Thinly sliced potatoes and onions are baked in a rich and cheesy sauce to make the Potatoes Au Gratin of your dreams! Slice the potatoes and onions thinly for an evenly baked and delicious side dish.
Peel the potatoes, then thinly slice, about ⅛-inch thick. Peel, halve and thinly slice the onion. Shingle the potato and onion slices in layers in a 2-quart to 3-quart casserole dish, seasoning each layer lightly with salt and pepper as you go.
In a medium pot, melt the butter over medium heat. Whisk in the flour. Cook whisking constantly, for 2 minutes. Drizzle in the warm milk while whisking. Continue whisking until the mixture starts to bubble and thickened slightly. Remove from heat and stir in the shredded cheeses, salt, and pepper until the cheese is melted. Pour the cheese sauce over the potatoes. Cover with foil.
Bake for 1 hour then remove foil and bake for an additional 20 to 30 minutes or until the top is golden brown and the potatoes are tender.
Notes
Cut thin and even potato slices. This will help the potatoes to cook evenly and thoroughly. I recommend using a mandoline or sharp knife to create uniform ⅛-inch thick slices.
Use warm milk. For the best results, heat the milk on medium heat in the microwave or low heat on the stove until it is 100-110 degrees Fahrenheit before using it in the recipe. This will help the cheese sauce come together more easily and is less likely to clump the roux, resulting in a silky smooth sauce.
Bake the casserole for the full cooking time. The most common issue when making au gratin potatoes is not cooking the potatoes long enough; so be sure to cook them the full time and to test the potatoes for tenderness with a sharp knife before removing from the oven.
Remove the aluminum foil towards the end of baking. Or for the final 20-30 minutes. Cooking the potatoes covered allows the potatoes to steam and soften, but uncovering at the end will allow them to brown up nicely and for the cheese sauce to thicken up to be perfectly velvety.
Potato au gratin variations. Top with buttery bread crumbs, add fresh thyme or other herbs to the sauce, sprinkle the top of the potatoes with extra cheese or try using an aged or smoked cheese like Gouda or fontina in the sauce, swap red onion instead of white, or use a mixture of sweet potatoes and russet potatoes.