Preheat your oven to 350°F.
In a large mixing bowl, combine the mashed potatoes, half of the cheddar cheese, crumbled bacon, sour cream, milk, green onions, garlic powder, salt, and black pepper. Mix until well combined and creamy.
Transfer the potato mixture into a greased 9x13-inch baking dish, spreading it evenly.
Sprinkle the remaining cheddar cheese on top of the potato mixture. If desired, add a sprinkle of paprika for color.
Bake in the preheated oven for 25-30 minutes, or until the cheese is melted and bubbly.
Let the casserole cool for about 5 minutes before serving.