In a large mixing bowl, combine the ground beef, breadcrumbs, chopped onion, minced garlic, egg, Worcestershire sauce, Dijon mustard, dried thyme, salt, and pepper. Mix until well combined.
Shape the mixture into oval-shaped patties, about 1/2 to 3/4 inch thick.
Heat the olive oil in a large skillet over medium-high heat. Once hot, add the patties to the skillet (you may need to do this in batches to avoid overcrowding) and cook for about 4-5 minutes on each side, or until browned and cooked through. Transfer the cooked patties to a plate and set aside.
In the same skillet, reduce the heat to medium. Add the sliced onion and mushrooms (if using) and cook, stirring occasionally, until softened and lightly browned, about 5-7 minutes.
Push the onions and mushrooms to one side of the skillet and melt the butter on the other side. Sprinkle the flour over the butter and cook, stirring constantly, for 1-2 minutes to make a roux.
Gradually pour in the beef broth, stirring constantly to prevent lumps from forming. Bring the mixture to a simmer and cook for a few minutes until the gravy thickens.
Season the gravy with salt and pepper to taste. Return the cooked Salisbury steaks to the skillet, spooning some of the gravy over the top.
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