Prepare the Patties: In a large mixing bowl, combine the ground beef, breadcrumbs, milk, chopped onion, egg, Worcestershire sauce, garlic powder, salt, and pepper. Mix until just combined—don’t overwork the meat. Divide the mixture into four equal portions and shape them into oval patties.
Cook the Patties: Heat the olive oil in a large skillet over medium heat. Once hot, add the patties and cook for 4-5 minutes per side, or until browned and cooked through. Remove from the skillet and set aside.
Make the Gravy: In the same skillet, add the butter. Once melted, toss in the mushrooms and sauté until tender, about 3-4 minutes. Sprinkle the flour over the mushrooms, stirring to coat, and cook for an additional minute to form a roux.
Simmer: Gradually add the beef broth, soy sauce, thyme, and Dijon mustard (if using). Stir well to combine, making sure no lumps form. Allow the gravy to simmer for 3-5 minutes, or until it thickens slightly.
Return the Patties: Add the cooked Salisbury steaks back into the skillet, spooning some of the gravy over the top. Let them simmer in the gravy for an additional 5 minutes to absorb the flavors and ensure everything is heated through.
Serve: Once the patties are fully coated in the rich mushroom gravy, remove from heat. Serve with mashed potatoes, rice, or a side of vegetables.