In a medium sized bowl, combine ground beef, ½ of the onions, breadcrumbs, egg, 1 tbsp Worcestershire sauce, onion soup mix and pepper.
Shape into 6 oval patties. Spray a large nonstick skillet with an olive oil mister or non-fat cooking spray and set over medium high heat. Brown both sides of patties, about 3-4 minutes each side.
Remove patties from pan, and then return pan to heat. Add in remaining onions and mushrooms, and 1 tbsp of the beef broth, and cook for about 2-3 minutes. Return the Salisbury steak patties to the pan.
In a small bowl, whisk together remaining beef broth, remaining Worcestershire sauce, ketchup, flour, and mustard powder.
Pour into skillet over the meat, turn heat down to medium-low, cover and cook for about 20 minutes.