Step 1

Preheat oven to 350 degrees F (175 degrees C). Spray a 9x13-inch baking dish with cooking spray.

Step 2

Melt butter in a saucepan over medium-low heat; cook and stir onion until softened, 5 to 10 minutes. Add mustard, poppy seeds, and Worcestershire sauce; cook and stir for 5 minutes.

Step 3

Arrange the bottoms from each roll in the prepared baking dish. Spoon 2/3 the onion mixture over the roll bottoms. Add ham and Swiss cheese to each roll. Put tops of rolls over the Swiss cheese layer. Brush the remaining 1/3 onion mixture over tops of rolls. Cover dish with aluminum foil.

Step 4

Bake in the preheated oven for 15 minutes. Remove aluminum foil and bake until tops of rolls are lightly browned, 5 to 10 minutes.