Preheat the oven to 350 degrees F (175 degrees C).

Mix together onion, butter, poppy seeds, mustard, and Worcestershire sauce in a medium bowl until well combined.

Slice rolls in half horizontally and set aside tops. Spread butter mixture onto bottoms, then top with ham and Swiss cheese. Cover with tops and arrange sandwiches in a single layer in a baking dish.

Bake in the preheated oven until rolls are lightly browned and cheese is melted, 10 to 12 minutes.