Preheat oven to 325°. In a small mixing bowl mix onion, mustard, poppy seeds, and melted butter. Leaving the dinner rolls intact, slice them open so that you have one solid top and bottom. Place the bottom half on a baking sheet or in a 9×13 inch baking pan, the pan size will depend on choice of rolls. Line the bottom half of the rolls with cheese slices and ham. Return the top part of the rolls and drizzle the butter mixture evenly over top. Cover with foil and bake for 20 minutes. Now use a sharp knife and separate the little sandwiches. Place on a pretty platter and serve immediately.
Notes from Big Red Kitchen- These can be made ahead of time and refrigerated, may require 5 more minutes of baking. Also can be prepared ahead and frozen. Defrost in refrigerator overnight and bake as directed. I love keeping a pan of these in the freezer for emergencies. Leftovers can be reheated in the microwave for about 15-20 seconds.