In a large mixing bowl, combine the ground beef, egg, Worcestershire sauce, Dijon mustard, minced garlic, breadcrumbs, onion powder, salt, and pepper. Mix until just combined.
Divide the mixture into four equal portions and shape into oval patties, about 1/2 inch thick.
Heat olive oil in a large skillet over medium-high heat. Sear the patties for 4-5 minutes on each side until browned. Remove and set aside.
In the same skillet, sauté the onions and mushrooms for 5-7 minutes until translucent. Add butter and melt.
Sprinkle flour over the onion and mushroom mixture, stir for 1 minute, and gradually whisk in beef broth, Worcestershire sauce, and garlic powder. Cook until thickened.
Return patties to the skillet, nestle into the gravy, reduce heat to low, and simmer for 5-10 minutes.
Serve the Salisbury steak with mashed potatoes, rice, or pasta.