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