In a large pot, boil the potatoes until tender. Drain and mash with the butter, sour cream, cheddar cheese, mozzarella cheese, bacon, and green onions. Season with salt and pepper.
Spread the mixture into a 9x13 inch baking dish.
Bake for 45 minutes in the preheated oven, or until the top is golden brown.
Interesting Facts
The dish can be prepared ahead of time and refrigerated until ready to bake.
This casserole is great served with a salad or roasted vegetables.
Leftovers can be stored in the refrigerator for up to 3 days.