In a large mixing bowl, combine the ground beef, breadcrumbs, milk, chopped onion, minced garlic, Worcestershire sauce, egg, salt, and pepper. Mix until just combined, being careful not to overwork the meat.
Shape the mixture into four oval patties, about 1 inch thick.
Heat 2 tablespoons of olive oil in a large skillet over medium-high heat. Once hot, add the patties to the skillet and cook for about 4-5 minutes on each side, or until browned and cooked through. Remove from the skillet and keep warm on a baking sheet.
In the same skillet, add 1 tablespoon of olive oil and the sliced mushrooms. Cook for 3-4 minutes until softened. Add the chopped onion and cook for another 2 minutes.
Sprinkle the flour over the mushrooms and onions, stirring to combine. Slowly pour in the beef broth while whisking to prevent lumps. Bring the mixture to a simmer and allow it to thicken for about 5 minutes.
Stir in the Worcestershire sauce, and season with salt and pepper to taste.
Serve the Salisbury steaks topped with the mushroom gravy, and garnish with chopped parsley.