Place ground beef, 1/3 cup condensed French onion soup, bread crumbs, egg, parsley flakes, salt, and black pepper in a mixing bowl.
Combine ingredients.
Shape beef into into 4 oval patties.
In a small bowl, whisk together the flour and remaining soup until there a no more lumps of flour in the soup.
Stir in the ketchup, water, Worcestershire sauce, sliced mushrooms, and mustard. Set aside.
In a large skillet over medium-high heat, brown both sides of the meat patties, until the meat is cooked through. Drain off any excess fat.
Pour reserved sauce over the cooked meat in the skillet. Bring sauce to a simmer and then cover and reduce heat to low.
Cook for 5-10 minutes, stirring occasionally, being careful not to break up the beef patties. Just move them around the pan gently turning them over with a spatula so they don’t burn.
Remove lid and simmer for an additional few minutes to thicken sauce if needed, or add water to thin out the sauce if it gets too thick.
Serve the beef patties nestled in a bed of creamy mashed potatoes topped with some of the delicious mushroom gravy sauce and a side of your favorite vegetables.