In a mixing bowl, combine ground beef, chopped onion, breadcrumbs, milk, beaten egg, Worcestershire sauce, garlic powder, salt, and pepper. Mix gently until just combined.

Divide the mixture into four equal portions and shape into oval patties about ½ inch thick. Set aside on a plate.

Heat vegetable oil in a skillet over medium-high heat. Cook hamburger steaks for 4-5 minutes per side until browned and internal temperature reaches 160°F.

Once cooked, set the patties aside. In the same skillet, sauté sliced mushrooms for 3-4 minutes until tender.

Add butter to the skillet and melt. Whisk in flour to create a roux, cooking for 1-2 minutes until light golden.

Gradually whisk in beef broth and additional Worcestershire sauce. Cook and stir until gravy thickens.

Return hamburger steaks to the skillet and spoon gravy over. Heat together for 2-3 minutes.

Serve hot over mashed potatoes or rice, drizzling extra gravy on top.