Place the minced beef in a frying pan or skillet and brown it quickly over high heat, stirring often. Drain off the juices and sprinkle in the flour. Stir well then add the onion soup. Cook, stirring often, until the meat mixture is thickened and most of the liquid has been absorbed. Test for seasoning and add salt and freshly-ground black pepper if desired.
Peel and slice the onion. Spread mustard and mayonnaise to taste on the buns. Spoon the minced beef mixture onto the buns and top with onion rings. Serve immediately.