Boil potatoes in lightly salted water until tender.Drain,reserving the liquid,and set them aside to cool Using the reserved potato water,boil the cabbage and onion for about 5 minutes- Drain and set aside.When potatoes are cool enough to handle,peel and mash with milk,butter,and salt and pepper,to taste.Add the reserved onion and cabbage mixture.Mix 2/3 of the cheese with the potatoes and transfer to a greased casserole or shallow baking dish.Sprinkle with remaining cheese on top.Heat in a moderately hot oven and let cheese on top brown slightly.