Preheat oven to 325 F.
Slice your rolls in half horizontally, but do not slice completely through.
In a small saucepan, melt the butter.
Whisk in the onion, mustard, poppy seeds, and Worcestershire sauce.
Open the roll, and brush the butter mixture evenly over both the top and bottom half of the rolls.
Layer the ham and Swiss slices over the bottom half of the rolls.
Close the rolls, and cover with foil.
Bake for about 15 minutes, or until cheese is melted.
Remove from oven, and serve warm.