Gradually add milk, whisking constantly, until the sauce thickens (5-7 minutes).
Remove from heat and stir in 1½ cups cheddar and ½ cup Parmesan cheese until smooth. Season with garlic powder, onion powder, smoked paprika (if using), salt, and pepper to taste.
Layer potato slices in the prepared dish, overlapping slightly. Pour a thin layer of cheese continue reading …