Slice each bun in half, or take a serrated knife and cut the entire loaf of Hawaiian rolls horizontally. Lay bottom half in a 9″ square pan. Spread roll bottoms with horseradish sauce. Layer with ham and cheese. Replace tops. In a small skillet, heat butter over medium high heat. Add onion, cooking and stirring for about 3 minutes, or until tender. Add brown sugar, mustard, poppy seeds, Worcestershire sauce, and garlic powder, mixing until sugar has dissolved. Pour over rolls. Refrigerate, covered, for several hours or overnight. Bake at 350ºF covered, for 25 minutes. Uncover and bake 8-10 minutes longer, or until golden-brown on top.