One of THE VERY BEST Salisbury Steak

In a large bowl, mix ⅓ cup of the French onion soup with ground beef, breadcrumbs, egg, salt, and pepper. Shape into 6 oval patties.
Heat a large skillet over medium-high and brown patties on both sides. Drain any excess fat.
In a small bowl, whisk remaining soup, flour, ketchup, Worcestershire sauce, mustard powder, and water until smooth.
Pour the sauce over the patties in the skillet, cover, and cook for 20 minutes, stirring occasionally.