Boil potatoes in salted water until tender.
While potatoes are cooking, sauté onions in 1 tablespoon butter until soft.
Drain potatoes; chunk up the potatoes in the pot by mashing them with your spoon.
Add onions, sour cream, chives, 1 cup cheese, and remaining 1⁄2 cup butter.
Salt and pepper to taste.
Transfer to a 9x13-inch casserole dish and cook 45 minutes at 350°.
Top with remaining 11⁄2 cups cheese and return to oven just until cheese is melted.