In a large bowl, combine the ground beef, breadcrumbs, finely chopped onion, milk, egg, Worcestershire sauce, garlic powder, salt, and pepper. Mix gently until just combined. Set aside for about 5 minutes.

Using slightly wet hands, shape the mixture into four oval patties, each about 1 inch thick.

Heat a large skillet over medium heat and add olive oil. Cook the patties for 4-5 minutes on each side until browned. Remove and keep warm.

In the same skillet, sauté the chopped onion and sliced mushrooms for about 5 minutes until tender.

Pour in the beef broth and Worcestershire sauce, increase heat and bring to a gentle simmer for around 2 minutes.

Mix cornstarch with water until smooth, then whisk into the gravy. Cook for 2-3 minutes until thickened.

Return the cooked patties to the skillet, spoon gravy over them, and let simmer together for about 5 minutes.

Serve the Salisbury steaks hot with gravy over the top alongside your favorite sides.