8 slices cooked roast beef, 1/8 inch thick
1/4 tsp. salt
1/4 tsp. pepper
3 tbsp. butter
1-2 onions, thinly sliced
1 tbsp. flour
1 1/2 c. leftover roast beef gravy mixed with beef broth
1 tbsp. vinegar
1 1/2 c. Cheddar or Swiss cheese, grated

Preheat oven to 425 degrees. Arrange beef in 9x13 inch ungreased baking dish, overlapping slices. Sprinkle with salt and pepper. Melt butter in 12-inch skillet over moderate heat. Add onions and cook, covered, 10 minutes. Stir in flour and cook uncovered, 1 minute, stirring.

Gradually stir in gravy into skillet, cook 3-5 minutes until slightly thickened. Stir in vinegar and pour over beef. Sprinkle with cheese and bake uncovered 15 minutes.