Preheat oven to 375˚F and grease a 9x13 inch baking dish.
If using fresh potatoes, place in a saucepan with cold water. Simmer 12-14 minutes or until fork tender. Drain and allow to cool slightly.
Melt butter, add sour cream, soup, onion and 1 ½ cups cheese.
Toss in the potatoes (or frozen hash brown potatoes) and combine. Spread into a prepared baking dish.
Top with remaining cheese and bake for 28-30 minutes or until browned and bubbly.
