In a large mixing bowl, combine the ground beef, breadcrumbs, Parmesan cheese, egg, garlic powder, salt, and pepper. Mix gently until just combined, avoiding overmixing to keep the patties tender. Divide the mixture into 4-6 equal portions and shape each into a patty about ¾-inch thick. Set aside.

Heat 1 tablespoon of olive oil in a large skillet over medium-high heat. Add the patties and sear for 3-4 minutes on each side until golden-brown. Remove the patties and keep warm.

In the same skillet, add the remaining tablespoon of olive oil. Add sliced mushrooms and finely sliced onion, sautéing for about 5 minutes until softened and golden brown.

Deglaze the pan with beef broth, scraping up browned bits. Stir in Worcestershire sauce and mix any flour with water to create a slurry, then add and whisk. Simmer for 5-7 minutes until gravy thickens.

Return the patties to the skillet, spoon gravy over them, and cover. Simmer on low heat for about 10 minutes.

Plate the Salisbury steaks and drizzle mushroom gravy over. Serve with sides like mashed potatoes or steamed vegetables.