Preheat oven to 350 degrees. Melt butter in a pan; add mustard, Worcestershire sauce, minced onion and poppy seeds. Tear chipped ham into small pieces and mix into ingredients in pan. Divide mixture evenly among the buns and top with a slice of Swiss cheese. Wrap in foil. Bake for 20 minutes or until piping hot. Sandwiches can be frozen and heated directly from freezer by extending baking time to 30-40 minutes.