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