Place potatoes in pot covered with salt water and bring to boil. Reduce heat and simmer until potatoes are tender with pierced fork. Drain

While potatoes are cooking, preheat oven to 350 F (175C) and butter casserole dish

Combine drained potatoes, cottage cheese, sour cream and onion in a bowl. Transfer to prepared casserole dish. Top with cheddar cheese.

Bake until bubbly and golden brown, 30-40 mins.