Mashed potatoes

1. Chop the potatoes into 2" pieces and put them in a large pot. Cover them with water by one inch and add 2 teaspoons salt. Bring to a boil and continue to boil until the potato is easily pierced with a fork, about 10-15 minutes.
2. When the potatoes are cooked, drain the water and transfer them to a large bowl. Add the butter, milk, 1 teaspoon of salt, and a pinch of pepper. Mash the potatoes using hand beaters or a potato masher until smooth. If the potatoes finish cooking before the meat mixture is ready, keep them in a warm spot and cover the bowl with plastic wrap to maintain their warmth and creaminess.

Beef filling

1. Preheat the oven to 400°F.
2. Warm a large, high-sided pan over medium heat, then add the butter and olive oil. When the pan is heated, add the onion, carrots, celery, garlic, 1/2 teaspoon of salt, and 1/4 teaspoon of black pepper. Stir and cook until the onions start to get some color, 7-10 minutes.
3. Push the vegetables to the side and drop pinches of ground beef into the pan, then season with 1 teaspoon of salt and 1/2 teaspoon of black pepper. Cook, breaking up the meat with a spatula as it browns.
4. When the beef is fully cooked, add the tomato paste and Worcestershire sauce. Stir everything together and let it cook for a few minutes, then add the beef broth. Bring the mixture to a boil. (If you're using peas, add them now.)
5. Combine the cornstarch and water. When the beef mixture boils, remove it from the heat. Quickly stir in the cornstarch slurry. The mixture will thicken quickly.
6. Pour the thickened beef filling into a casserole dish. Top with the mashed potatoes by dropping large spoonfuls all over the top and gently seal the mash by smoothing it with the back of the spoon all the way to the edges of the dish. You can run a fork along the potatoes to create more edges for crispy browning.
7. Bake for 30 minutes or until the brown gravy bubbles at the edges and parts of the potato mash have browned.