Preheat oven to 350 degrees. Coat a casserole dish with cooking spray.
In a large mixing bowl, combine mashed potatoes, 3/4 cup cheddar cheese, sour cream, milk, butter, fine sea salt, ground black pepper and garlic powder. Stir well and pour into prepared casserole dish.
Top with remaining 3/4 cup shredded cheddar cheese.
Bake for 15 minutes.
Remove and top with bacon crumbles and scallions. Serve immediately.
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