In a mixing bowl, COMBINE mushroom soup with tuna; ADD pepper and onion; MIX well.
SPOON equally onto hamburger buns; TOP with slice of cheese.
BRUSH tops of buns with melted butter.
BAKE on cookie sheet at 400° until cheese melts and buns are golden brown, about 10 minutes.
Per Serving (excluding unknown items): 968 Calories; 65g Fat (60.7% calories from fat); 56g Protein; 38g Carbohydrate; 2g Dietary Fiber; 191mg Cholesterol; 3311mg Sodium. Exchanges: 2 1/2 Grain(Starch); 7 Lean Meat; 0 Vegetable; 9 1/2 Fat.