Boil potatoes in salted water until soft. Drain and set aside.
Save cooking water. Add onion and cabbage to potato water. Boil until tender. Drain and set aside.
Mash potatoes adding 1/2 cup heavy cream and 2oz. butter. Add onion and cabbage to potatoes. Add 2/3 of cheese to potato mixture. Mix well and place in a casserole dish. Sprinkle with remaining cheese.
Heat casserole in 350F degree oven until cheese is browned.