Indulge in a mouthwatering grilled flat iron steak sandwich layered on sliced ciabatta bread and accompanied by a tangy sauce, sweet griddled onions, and crispy greens. It's the ultimate choice for a truly satisfying meal.
Cooking steak well can be a little nerve-wracking, especially with an expensive cut. What's the best seasoning? Should I marinate it or not? How long will it take to get to the right temperature? Should I cook it quickly on high heat or slow and low?
Look no further! Here's a foolproof recipe for a delicious flat iron steak sandwich with griddled onions. With simple seasoning, precise cooking time, and a few carefully chosen ingredients to complement the perfectly grilled steak, you'll be enjoying the best sandwich you've had in a while.
Read on for all the loving details for making this Flat Iron Steak Sandwich.
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Featured Ingredients
Read below about a few of the ingredients used to make Flat Iron Steak Sandwiches with Griddled Onions.
For the sauce
For the sandwich
Beef - For this sandwich, the flat iron cut of beef works best. It's naturally tender and easy to cook using this method. All you need is salt for seasoning. If you like to use pepper on your steaks, add it after cooking so it doesn't burn at the high grill temperature. Depending on your meal plan, you can either salt the steak 1-4 hours before cooking or just before going on the heat.
Relish - This is very important. You can't use any old relish in this sauce! It specifically needs hot dog relish, which has mustard and other seasonings that standard relish does not.
Bread - Ciabatta is an ideal choice for a messy steak sandwich. It provides a sturdy base for the saucy and heavy ingredients without overwhelming the flavor. A rustic white or wheat bread would also work well.
Greens - For balanced flavor, it's best to choose slightly bitter and peppery greens. You can try arugula, baby kale, frisee, or mizuna. You can even mix them up for a blend of flavors. The earthiness of the greens complements the tangy sauce, sweet onions, and rich, savory steak in the best ways. Here's more on all types of greens and how to use them!
Tools for the job
- Sharp knife (paid link) and cutting board
- Measuring cups and spoons
- Grill
- Thermometer
- Tray and foil
- Tongs, stirring spoon, small whisk
- Skillet
How to make Flat Iron Steak Sandwiches
Pre-prep ingredients ahead of time
This is optional, but really makes a difference in flavor if you have the time!
- Salt the steak with approximately 1 teaspoon of salt per pound and set it on a rack with a pan under to catch moisture. Refrigerate the steak for 1-4 hours ahead of cooking. NOTE: If you do not have time to salt it earlier in the day, salt it right before it goes on the grill.
- Make the sauce. Add all of the sauce ingredients to a small bowl and whisk together. Cover and refrigerate so the flavors have time to come together. The longer this sauce sits, the better it gets!
Griddle the onions
This can be done before you grill the steak.
- Thinly slice the onions.
- Then, warm a large nonstick pan over medium-high heat and add 1 tablespoon of oil.
- When the oil moves freely in the pan, add the onions. Stir them occasionally, but not too often, so the edges will start to brown and the onions will soften. This takes about 10 minutes. I like to get lots of seared pieces mixed in with the sauteed onions for added flavor.
- Set the onions aside, but don't clean the pan. Melt a few tablespoons of butter in the pan and toast the bread until golden brown. Keep a close eye on it, as it can quickly burn.
Grilling the Flat Iron steak
The steak cooks at a very high heat on the grill. To prevent a fire, ensure your BBQ grates are clean before heating.
- Take the steak out of the refrigerator right before you plan to grill it. This will allow you to sear the outside well while keeping the insides tender and medium rare.
- Pat the flat iron steak dry, then rub it with 1-3 tablespoons of olive oil. Lay it over direct heat on the grill, pressing a little to ensure all the meat has contact with the hot grates.
- Leaving the lid open, grill the steak for 4 minutes and flip it, pressing lightly again to ensure the meat has contact with the hot grates. Cook for another 4 minutes.
- At this point, you should have nice sear marks on both sides, and the steak should be getting close to the correct temperature. We like to take it to a nice medium rare by pulling it from the heat at 130°F.
See the chart for the perfect temp!
The temperature will continue to rise after it's been pulled from the heat, so remove it when the stead is about 5 degrees cooler than you want it to be.
When the steak is done to your preference, remove it from the heat and let it rest tented with foil leaving at least 1" space between the steak and the foil.
After grilling the steak
When the steak is done to your preference, remove it from the heat and let it rest tented with foil. To do this, wrap the entire dish or seal foil only over the top, leaving at least 1" space between the steak and the foil.
Why rest the steak? Slicing into any meat right after cooking will cause the juices near the surface to spill out, making the meat dry and less flavorful. Letting the meat rest for a few minutes allows the natural juices to redistribute throughout the steak, resulting in a more tender and juicy cut that is worth savoring.
- After the steak has rested for 10 minutes, transfer it to a cutting board. Using a sharp knife (paid link), cut slices as thin as you can manage, going against the grain. For the flat iron steak, you'll be cutting it along the width rather than the length.
Hot Tip!
It is best to check the temperature with a thermometer for accuracy. If you do not have one, these cooking times should be fairly accurate since most flat iron steaks are similar in size. I found they range from 1 -2 pounds.
Typically, this cut of beef is a bit thicker on one end, so I will cut it in half so the thinner side doesn't overcook. Although, it's only the difference of 1-2 minutes per side.
Assembling the Flat Iron Steak Sandwich with Griddled Onions
- Slather a good helping of the sauce on both sides of the bread.
- Next, layer 4 to 5 slices of steak on the bottom bun with ¼ cup of griddled onions and a good helping of greens.
- Add the top bun and slice in half with a serrated knife.
- Enjoy your sandwich alongside a green salad, some chips, or a second steak sandwich!
Variations and substitutions for the Sandwich
- You can substitute hanger steak for the flat iron steak
- Another great addition to this sandwich is cheese! Soft cheese like gorgonzola or goat cheese is perfect for this sandwich because it doesn't need to melt to be soft.
- Instead of using the zesty sauce listed in the recipe, go for something bright and green, like chimichurri. Here is a great recipe to try for chimi. Then add sliced bell peppers for an added crunch.
Leftover Steak Sandwich
- You can refrigerate leftover steak for up to 3 days. Then, reheat it in a nonstick pan on the stove on low until warmed through.
- Refrigerate leftover sandwich sauce in a sealed container for up to 2 weeks.
Food Safety
- Wash hands after touching raw meat
- Do not use the same utensils on cooked food that previously touched raw meat
- Don't leave food sitting out at room temperature for extended periods
- Never leave cooking food unattended
- Be safe while using a grill or BBQ
- See more guidelines at USDA.gov.
More sandwiches and burgers
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Flat Iron Steak Sandwich with Griddled Onions
- Total Time: 4 hours 25 minutes
- Yield: 4 servings
Description
Indulge in a mouthwatering grilled flat iron steak sandwich layered on ciabatta bread and accompanied by a tangy sauce, sweet caramelized onions, and crispy greens.
Ingredients
- 1 lb flat iron steak
- ¼ cup olive oil, divided
- 1 large onion
- salt and pepper
- 4 ciabatta buns or other rustic bread
- salted butter for the bread
- 2-4 cups textured greens like arugula, mizuna, frisee, or baby kale
Sauce ingredients
- ⅔ cup mayonnaise
- 2 tablespoons barbeque sauce
- 2 tablespoons hotdog relish
- 2 teaspoons Anything Seasoning Blend
- 1 Tablespoon Worcestershire sauce
- 2 -4 teaspoons hot sauce like Cholula
Instructions
Make the sauce
- Add the mayonnaise, barbecue sauce, hotdog relish, and Anything Seasoning Blend to a small bowl and whisk together.
- Cover and refrigerate so the flavors have time to come together. It’s best to do this a few hours before making the sandwiches.
Griddle the onions and toast the bread
- Warm a large nonstick pan over medium high heat and add 1 tablespoon of oil. When the oil moves freely in the pan, add the onions. Stir them occasionally, but not too often, so the edges will start to brown and the onions will soften. This takes about 10 minutes. Set the onions aside until it's time to make the sandwiches.
- While the onion pan is still hot, add a few tablespoons of butter. Once it has melted, put the sliced bread into the pan to toast one side only. Add more butter to the pan until all the bread has been toasted.
Grill the steak and assemble the sandwich
- Salt the steak with 1 teaspoon of salt per pound and set it on a rack with a pan under to catch moisture. Refrigerate the steak without covering for 1-4 hours before cooking. If you do not have time to salt it a few hours in advance, salt it right before it goes on the grill.
- Heat the grill to 500°F.
- Take the steak out of the refrigerator right before you plan to grill it, pat it dry, then rub it with 1-3 tablespoons of olive oil. Lay it over direct heat on the grill, pressing the edges to ensure all the meat has contact with the hot grates.
- Cook the steak for 4 minutes and flip it. Press to adhere again and cook for another 4 minutes. At this point, you should have nice sear marks on both sides, and the steak should be getting close to temp. Pull the steak off the heat when it gets to 5 degrees lower than your preferred finishing temperature.
- When the steak is done to your preference, remove it from the heat and let it rest, tented with foil. After it has rested for 10 minutes, transfer it to a cutting board. Cut slices as thin as you can manage using a sharp knife (paid link), going against the grain. For the flat iron steak, you'll be cutting it along the width rather than the length.
- Assembling the sandwiches: Slather a good helping of the sauce on both sides of the bread.
- Next, layer 4 to 5 slices of steak on the bottom bun with ¼ cup of griddled onions and a good helping of greens.
- Add the top bun and slice in half with a serrated knife. Enjoy your sandwich alongside a green salad, some chips, or a second steak sandwich!
- Prep Time: 10 minutes
- Dry Bringing Time: 4 hours
- Cook Time: 15 minutes
- Category: Main dish
- Method: Stove-top, grill
- Cuisine: American
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