Preheat the oven to 350F

Spray the casserole dish with cooking spray (including the sides)

Combine the hash browns, pepper, onion, soup, milk, sour cream, and cheese in the mixing bowl

Melt the butter in a small skillet

Pour the butter over the mixture

Stir the ingredients in the bowl until well-combined

Transfer to the casserole dish

Bake for one hour in the preheated oven