Peel (if needed) and slice your potatoes thinly. I love using a mandoline to make sure every slice is even, which helps them cook perfectly.
Melt butter in a saucepan, whisk in flour, then gradually add milk until smooth. Stir in salt, pepper, and half of the shredded cheese for a luscious sauce.
Arrange a layer of potato slices in your baking dish, pour over some sauce, then repeat. Sprinkle remaining cheese and paprika on top. Bake at 375°F (190°C) for about 60 minutes until the top is golden and bubbly.
Let the dish rest for 5–10 minutes after baking. This helps the sauce thicken slightly and makes slicing easier.