Heat the oven to 350°F and line a 9-by-13-inch pan with parchment.
Slice the attached rolls horizontally as one slab and set the bottom slab in the pan.
Layer the ham and Swiss evenly to the edges, then replace the top slab.
Whisk the butter, Dijon, Worcestershire, poppy seeds and onion powder and brush it over every roll.
Cover with foil and bake for 15 minutes, then uncover and bake for 5 to 7 minutes until the centers reach 165°F and the tops are brown.
Rest for 5 minutes and cut through the seams into 12 sliders.