Preheat the oven to 375°F or 190°C and grease a baking dish. Pat the hash browns thoroughly dry with paper towels.
Spread the hash browns across the bottom of the dish. Sprinkle the bacon, bell pepper, green onions, and most of the cheese on top.
Whisk the eggs, milk, sour cream, garlic powder, salt, and pepper until no large sour cream lumps remain.
Pour the egg mixture evenly over the casserole. Finish with the remaining shredded cheese.
Bake for 45 to 50 minutes, until the middle is firm and the edges are lightly browned.
Let the casserole rest for 10 minutes before cutting. Cool extra slices completely before freezing.