In a bowl combine ground beef, 1/3 cup French onion soup, bread crumbs and egg. Mix by hand until well mixed. Form into 6 patties.

Fry patties the patties in a skillet over medium-high heat until browned on both sides. Patties may still be pink in the middle, they will continue cooking while simmering. 

Remove to a plate. Leave grease in the pan for cooking onions and mushrooms.

Add the onions to the pan and cook stirring frequently until wilted and golden.

Add mushrooms to the onions in the hot pan and cook until slightly softened.

Combine remaining soup, ketchup, Worcestershire sauce, mustard, and flour. Whisk well to get rid of all flour lumps. 

Add mixture to mushrooms in the skillet. Reduce heat to low and bring to a simmer. 

Add beef patties back into pan and spoon gravy over top of the patties. 

Cover and simmer over low until is liquid thickened, about 30 minutes.

Serve with rice, noodles, noodles or mashed potatoes.