Preheat the oven to 350 degrees F. Spray a 9x13 baking dish and set aside.
Peel and slice potatoes into large coins, about an ⅛ inch thick.
6-8 potatoes
In a saucepan, over medium heat, melt 3 tablespoons of butter. Add in onion and cook until translucent. Add in the flour and whisk to combine. Add the milk and cream and heat to a low boil and stir for one minute. Add garlic powder, salt, and pepper.
4 Tbsp butter, 1 onion, 3 Tbsp all-purpose flour, 1 1/2 cup milk, 1 1/2 heavy cream, 1 tsp garlic powder, 1 tsp salt, 1 tsp pepper
Layer potatoes in a baking dish, overlapping as necessary.
Pour cream sauce on top of the potatoes. Dot with 1 Tablespoon of butter, cut into pieces. Bake covered for 30 minutes.
Remove the cover and bake another 40 minutes, until the top is golden brown, and the potatoes in the center are tender. Let stand 5-10 minutes before serving.