Heat the oven to 375°F and grease a 9-by-13-inch baking dish.

Boil the potatoes in salted water for 12 to 15 minutes, until tender. Drain and mash with the butter, sour cream, milk, salt, and pepper.

Stir in 1½ cups of cheese, half the bacon, and half the green onions.

Spread the mixture into the dish. Top with the remaining cheese and bacon.

Bake for 20 minutes, then add the remaining green onions before serving.