Preheat oven to 325°. Mix margarine, mustard, onion, and poppy seeds together to make a sauce. Spread sauce on inside of top and bottom pieces of bun/bread for each sandwich. Add 2 slices of ham and 1 slice of cheese to each sandwich. Close sandwiches and wrap individually in foil. Heat for 20 minutes before serving. Sandwiches can be made ahead and frozen before cooking.
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