In a large mixing bowl, combine all ingredients for Salisbury Steak patties. Divide mixture into 6 balls and gently flatten to form patties.
Heat your skillet over medium heat and add 1 tablespoon butter. Once the skillet is hot, add patties to the pan. Cover and cook for about 5 minutes until bottoms are brown. Flip patties over, cover and cook for another 5 minutes until internal temperature is at least 165 degrees. Remove patties from pan and transfer to plate.
For the gravy: In the same pan, add butter, mushrooms, onions, red wine vinegar and Worcestershire sauce. Cover and cook for several minutes until onions and mushrooms are soft and caramelized.
Add salt, pepper, ketchup, mustard and beef broth to the mushroom mixture and bring to a low simmer for several minutes until broth lightly thickens. Spoon some broth into a small bowl and mix in cornstarch. Stir mixture into the gravy.
Add salisbury steak patties back to the pan and spoon gravy on top. Cover and let patties heat back up in the gravy.
Serve with a sprinkle of fresh chopped parsley (optional). Enjoy!