In a large mixing bowl, combine the mashed potatoes, 3/4 cup of cheddar cheese, sour cream, milk, half of the crumbled bacon, and half of the green onions. Mix well.
Season with salt and black pepper to taste.
Transfer the mixture to a greased baking dish. Sprinkle the remaining cheddar cheese and bacon on top.
Bake in the preheated oven for 25-30 minutes, or until the cheese is melted and bubbly.
Garnish with the remaining green onions before serving.
Notes:
This casserole can be made ahead of time and refrigerated. Just add a few extra minutes to the baking time if baking from cold.