Step 1: In a large bowl, combine the ground beef, breadcrumbs, egg, grated onion, Worcestershire sauce, garlic powder, salt, and pepper. Mix by hand until just combined, being careful not to overwork the meat.
Step 2: Shape the mixture into 4 oval-shaped patties, each about 3/4 inch thick. Press a small indentation into the center of each patty.
Step 3: Heat olive oil in a large skillet over medium-high heat. Place the patties in the skillet and sear for 4-5 minutes per side until deeply browned. Remove the patties and set aside on a plate.
Step 4: In the same skillet, add the sliced mushrooms and onions. Sauté for 5-7 minutes until the onions are translucent and the mushrooms have browned.
Step 5: Sprinkle the flour over the vegetables and stir constantly for 1 minute to cook out the raw flour taste.
Step 6: Slowly pour in the beef broth while whisking or stirring constantly to prevent lumps. Bring the sauce to a simmer.
Step 7: Return the beef patties to the skillet, nesting them into the gravy. Reduce heat to low and simmer for 10 minutes until the sauce has thickened and the patties are cooked through.
Step 8: Taste the gravy and adjust seasoning with salt and pepper if needed. Serve hot over mashed potatoes.