In a large bowl, combine ground beef, egg, Worcestershire sauce, Dijon mustard, minced garlic, breadcrumbs, onion powder, salt, and black pepper. Mix gently with your hands and form into 4 patties.

Heat 2 tablespoons of olive oil in a skillet over medium-high heat. Add patties and sear for 5-6 minutes on each side until browned. Remove and keep warm.

In the same skillet, add onions and mushrooms. Sauté for 7-9 minutes until softened. Remove and set aside.

Reduce heat to medium-low and add butter. Once melted, whisk in flour to create a roux. Cook for 1-2 minutes, then gradually add beef broth, stirring continuously.

Return patties and sautéed onion-mushroom mixture to the skillet. Simmer for an additional 5-7 minutes until the patties are cooked through.

Serve the patties hot with mushroom gravy ladled over them.