1 1/2 inch thick sliced precooked ham
2 tbsp. prepared mustard
3 tsp. chopped onion
1 c. ketchup
5 slices cheese
Onion ring; optional

Place ham in shallow baking dish. Spread with mustard; sprinkle with onion. Bake in hot oven, 400 degrees, for 15 minutes. Remove from oven. Pour ketchup over ham. Arrange cheese on top. Bake 10 minutes.