1In a medium skillet over medium high heat, brown ground beef and drain.
2Return ground beef to skillet and sprinkle in 2-3 tbsp flour.
3Add onion soup (I also drain out remaining onions from remaining soup and add to beef mixture but this is up to you - I think it gives it more flavor), stir, and cook until meat mixture is thickened and most of the liquid absorbed.
4Take off heat
5Spread mayo and mustard on both slices of bread, spoon ground beef mixture on to bread and top with sliced onion rings.