In a large mixing bowl, combine the ground beef, breadcrumbs, minced onion, minced garlic, egg, Worcestershire sauce, Dijon mustard, salt, and pepper.
Mix until well combined.
Divide the mixture into 4 equal portions and shape each portion into an oval-shaped patty.
Heat the olive oil in a large skillet over medium-high heat.
Add the patties to the skillet and cook for 4-5 minutes on each side, or until browned and cooked through.
Remove the patties from the skillet and set aside.
In the same skillet, melt the butter over medium heat.
Add the sliced mushrooms and minced onion to the skillet.
Cook, stirring occasionally, for 5-7 minutes, or until the mushrooms are golden brown and the onions are softened.
Add the minced garlic and cook for an additional 1 minute.
Sprinkle the flour over the mushrooms and onions in the skillet.
Stir well to coat the vegetables with the flour.
Gradually pour in the beef broth and Worcestershire sauce, stirring constantly to prevent lumps from forming.
Cook the gravy, stirring occasionally, until it thickens, about 5-7 minutes.
Season with salt and pepper to taste.
Return the cooked Salisbury steak patties to the skillet, nestling them into the mushroom gravy.
Spoon some of the gravy over the patties.
Garnish with chopped fresh parsley, if desired.
Serve hot with mashed potatoes, rice, or your favorite side dishes.
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