Make the sauce

1. Add the mayonnaise, barbecue sauce, hotdog relish, and Anything Seasoning Blend to a small bowl and whisk together.
2. 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

1. 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.
2. 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

1. 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.
2. Heat the grill to 500°F.
3. 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.
4. 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.
5. 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, going against the grain. For the flat iron steak, you'll be cutting it along the width rather than the length.
6. Assembling the sandwiches: Slather a good helping of the sauce on both sides of the bread.
7. Next, layer 4 to 5 slices of steak on the bottom bun with 1/4 cup of griddled onions and a good helping of greens.
8. 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!