In a bowl, combine butter, onion, mustard and poppy seeds. Spread about 1 tablespoon over both halves of buns. Layer ham and cheese on the bottom halves; replace tops. Wrap each sandwich in foil. Bake at 350 degrees F for 6-10 minutes or until cheese is melted, or freeze for up to 2 months. To use frozen sandwiches: Bake at 350 degrees F for 30-35 minutes or until cheese is melted.