Make patties. In a large bowl, combine the ground beef, breadcrumbs, egg, milk, Worcestershire sauce, salt, and pepper. Mix lightly with your hands until just combined, then form into 6 equal, oval shaped patties, about ¾ inch thick.
2 pounds ground sirloin, 1/2 cup breadcrumbs, 1 egg, 1 Tablespoon milk, 1 Tablespoon Worcestershire sauce, 1 teaspoon Kosher salt, 1 teaspoon black pepper
Sear steaks. Preheat a heavy bottom skillet over medium high heat and add the oil and butter. Once melted, add the beef patties and brown completely on one side (about 5 minutes) before flipping and browning the opposite side.
1 Tablespoon avocado oil, 3 Tablespoons butter
Cook mushrooms. Move the seared patties to a plate and cover with foil to keep warm. Add 2 additional tablespoons of butter to the hot skillet. Add the sliced onions to the skillet and cook for 4-5 minutes, until softened and starting to brown. Stir in the sliced mushrooms and cook 2-3 more minutes.
1 medium yellow onion, 8 ounces sliced portobello mushrooms
Make gravy. Sprinkle the flour over the vegetables and stir to coat, cooking the flour for 1-2 minutes. Pour in the beef stock, tomato paste, and Worcestershire sauce. Bring the gravy to a boil while stirring regularly. Reduce heat and simmer until thickened, about 1-2 minutes. Season with salt and pepper to taste.
2 Tablespoons all-purpose flour, 2 cups beef stock, 1 Tablespoon tomato paste, 1 teaspoon Worcestershire sauce, 1/2 teaspoon Kosher salt, 1/2 teaspoon black pepper
Combine, simmer, and enjoy. Return the seared salisbury steaks to the pan, spooning the hot mushroom gravy over the top. Cover, reduce heat to low, and let steaks simmer in gravy for about 10 minutes. Garnish with parsley or thyme and serve with mashed potatoes or rice.