In a large bowl, combine the ground beef, breadcrumbs, grated Parmesan cheese, egg, garlic powder, salt, and pepper. Mix gently until just combined. Form the mixture into 4-6 patties, about ¾ inch thick. Set aside to rest.

Heat 1 tablespoon of olive oil in a large skillet over medium-high heat. Carefully add the patties, searing them for about 3-4 minutes on each side until golden-brown. Remove the patties and set aside.

In the same skillet, add another tablespoon of olive oil and reduce the heat to medium. Add the sliced mushrooms and sliced onions, cooking them for about 5 minutes until golden and softened.

Pour in the beef broth to deglaze the skillet, scraping the bottom. Stir in the Worcestershire sauce and bring to a gentle simmer. For thicker gravy, whisk flour and water, and stir it in. Allow to simmer for another 5-7 minutes.

Return the browned patties to the skillet, ensuring they are submerged in the gravy. Cover and reduce the heat to low. Simmer for about 10 minutes to meld flavors and ensure patties are cooked through.

Remove from heat and plate the Salisbury steaks covered with the gravy. Serve with mashed potatoes or steamed vegetables.