In a large mixing bowl, combine the ground beef, breadcrumbs, egg, grated onion, 1 tbsp Worcestershire sauce, garlic powder, 1/2 tsp salt, and 1/4 tsp black pepper. Use your hands to mix everything thoroughly but gently until just combined.

Divide the meat mixture into 8 equal portions. Shape each portion into an oval or oblong patty, about 3/4 inch thick. Place them on a plate.

Heat 2 tbsp olive oil and 2 tbsp butter in a large skillet over medium-high heat. Once shimmering, add the patties (in batches if needed) and sear for 3-4 minutes per side until deeply browned. Remove and set aside.

Reduce heat to medium. Add sliced mushrooms and sliced onion to the skillet. Cook for 5-7 minutes until softened and mushrooms have released liquid.

Sprinkle flour over the onions and mushrooms. Stir and cook for 1 minute to form a roux.

Gradually whisk in the beef stock, then add ketchup, Dijon mustard, and 1 tsp Worcestershire sauce. Season with salt and pepper.

Return the patties to the skillet, nestle into the gravy. Reduce heat to low, cover partially, and simmer for 8-10 minutes until patties are cooked through.

Garnish with parsley if desired and serve immediately over mashed potatoes, egg noodles, or rice.