Mix together:

1/2 c. butter
1/2 c. diced onion
1/4 c. mustard
1 tsp. poppy seed

Spread mixture on top half of hamburger bun. Place a goodly portion of thinly sliced, good baked ham on bottom portion. On top of ham, place slice of Swiss cheese. Wrap sandwiches in foil and bake in 350 degree oven for approximately 15 minutes until hot. All time family favorite.