Sauté sliced franks and chopped onion in oil until lightly browned.
Sprinkle in flour, stir till mixture is coated; then add catsup and water.
Cook 5 minutes. Add cheese. Heat 1 more minute until melted.
Spoon into buns. Wrap in foil.
Bake at 350 degrees or cook on grill or in toaster oven for 15 to 20 minutes.