Preheat the oven to 350 degrees F (175 degrees C). Line a 9-inch square baking pan with aluminum foil and spray with cooking spray.
Slice attached rolls in half widthwise with a serrated knife, keeping rolls connected, into a bottom and top 'slab.' Place bottom slab in the prepared pan.
Layer about half the ham evenly over the bottom slab of rolls. Layer Swiss cheese evenly on top, and arrange remaining ham over the cheese. Cover with the top slab.
Place butter in a microwave-safe bowl. Heat in the microwave until melted, about 1 minute. Add mustard, poppy seeds, Worcestershire sauce, onion, salt, and pepper. Whisk to combine.
Pour butter mixture evenly and slowly over the rolls. Spread over the tops using a spatula; some of the mixture will pool at the base of the rolls. Cover with aluminum foil and let stand at room temperature for 5 to 10 minutes.
Bake in the preheated oven until cheese has melted, about 20 minutes. Uncover and continue baking until browned as desired, 3 to 5 minutes more. Slice into individual sliders.