Spray a large slow cooker (I used a 6-quart crockpot) with nonstick spray.
In a small saucepan over medium heat, melt the butter. Add the onion and sautÃ© until tender. Stir in the garlic powder. Briskly whisk in the flour; cook for about 1 minute. Very slowly add the milk whisking constantly until thickened.
Reduce the heat to low and allow to simmer and thicken. Remove from heat and stir in the sour cream.
Pour the creamy base into the prepared slow cooker insert. Top with the hash browns and shredded cheddar cheese. Add salt and pepper and stir everything together.
Cover and cook on high for 2-3 hours. Remove the lid and allow to cook another 30 minutes to an hour to get crispier on top.