A very tasty dish that you can do ahead of time and pop into the oven at the last minute. Good for a buffet.
Preheat oven to 325 degrees F. Lightly coat a large casserole with non-stick cooking spray.
Boil potatoes in salted water until tender. Mash or put through a ricer. While still hot, add sour cream, cottage cheese, butter, onions, salt, and pepper. Mix well. Put mixture in prepared casserole and sprinkle Parmesan on top. Bake for 30 minutes. Before serving, garnish with parsley.