In a medium skillet over medium high heat, brown ground beef and drain.
Return ground beef to skillet and sprinkle in 2-3 tbsp flour.
Add 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.
Take off heat
Spread mayo and mustard on both slices of bread, spoon ground beef mixture on to bread and top with sliced onion rings.