In a small bowl, combine the butter, mustard, onion, poppy seeds and sugar. Place roll bottoms, cut side up, in an ungreased 15 x 10 x 1 inch baking pan. Top each with colby cheese, ham and mozzarella cheese. Drizzle with half of the butter mixture.
Replace roll tops. Drizzle with remaining butter mixture. Bake, uncovered, at 350 degrees for 10-15 minutes or until cheese is melted.