Boil potatoes in a large pot until fork-tender (about 10-15 minutes).
Allow potatoes to cool. When the potatoes are cool enough to handle, peel them and use the large holes on a grater to shred them.
Place grated potatoes in a large bowl. Stir in 2 cups of the cheese (set aside the remaining cup for later) and all of the sour cream. Add salt and pepper to taste.
Transfer potato mixture to a greased 1 ½ quart baking dish.
Bake, uncovered, for approximately 15 minutes. Remove from the oven, sprinkle with remaining 1 cup of shredded cheese, and return to the oven. Bake for an additional 10 minutes, or until cheese is melted and potatoes are heated through.
Garnish with sliced green onions just before serving.