Melt the butter in a skillet on medium heat. Add the onions, and cook for 5 minutes. Stir in the dijon mustard, Worcestershire sauce, and poppy seeds and cook until the onions are soft (about 10 more minutes).
Slice the rolls in half all at once and place the bottom half in a 9×13 casserole dish. Spread on ¾ of the onion mixture; add the ham, cheese, and top half of the buns.
Spread the remaining onion mixture on top of the buns. Cover with aluminum foil and bake in the preheated oven for 15-20 minutes. Remove from the oven, and slice down the seams to make individual sliders.