Preheat the oven to 350°F (175°C).
Wash, peel, and slice the potatoes into thin rounds.
In a large mixing bowl, combine sour cream, milk, garlic powder, salt, black pepper, and half of the cheddar cheese.
Fold the sliced potatoes into the sour cream mixture until coated well.
Layer half of the potato mixture in a greased baking dish, followed by a layer of bacon bits and green onions.
Top with the remaining potato mixture, pressing down gently to compact.
Sprinkle the remaining cheddar cheese and, if using, crushed cornflakes on top.
Cover the dish with aluminum foil and bake for 30 minutes, then remove the foil and bake for an additional 15-20 minutes.
Check for doneness by inserting a fork into the potatoes; they should be tender and easily pierced.
Allow to sit for 10 minutes before serving.
