Add potatoes and garlic to a large pot. Fill with water, so the potatoes are completely covered. Bring to a boil, and cook for 15-20 minutes, until potatoes are fork tender, Drain
Preheat oven to 350°
Mash potatoes. Stir in butter, sour cream, and heavy cream. Mash further if necessary to get to your desired consistency. Season with salt and pepper. Stir in 1 1/2 cups cheese, and bacon (reserve some for the topping)
Pour into 13×9 in baking dish. Sprinkle with remaining cheese and reserved bacon. Bake for 20-25 minutes until hot and cheese is melted
Sprinkle with green onions before serving