Salisbury Steak casserole is a warm and comforting savory dish. It has a thick beef gravy with tender mushrooms and seasoned hamburger topped with rich, creamy potatoes.
Preheat the oven to 375 degrees Fahrenheit. Heat a large oven-safe skillet over medium-high heat on the stove.
Add the ground beef to the skillet, season with salt and pepper, and cook until mostly browned. It will finish cooking later.
Remove the cooked beef. Add the butter, diced onion, and mushrooms, and cook until tender, about 3-4 minutes.
Add the garlic to the skillet and cook for just a minute. Whisk the flour into the melted butter and vegetables to create a paste.
Slowly whisk in the beef broth to create the smooth gravy sauce.
Stir in the cooked meat, Worcestershire sauce, and vinegar. Remove the skillet from heat.
Spread the potatoes evenly over the skillet, top with the cubed butter, and bake for 15-20 minutes, until the potatoes are golden brown on top and the casserole is heated through.
Remove from the oven, and let it rest for a few minutes before serving.