In a small bowl, combine apple
butter and mustard. Spread 1 tablespoon mixture on each muffin half.
In another small bowl, combine ham,
onion, and cheese. Spread each muffin half with this mixture.
Place muffin halves on a baking
sheet and broil in the oven until the topping is bubbly. (Both mixtures may be prepared
in advance and stored separately for a quick breakfast.)