Au Gratin Potatoes Recipe

Preheat and prepare

Preheat oven to 375°F (190°C). Lightly butter a 9x13-inch (or equivalent) baking dish.

Peel (optional) and very thinly slice the potatoes to about 1/8-inch thickness using a mandoline or a sharp knife. Place slices in a large bowl of cold water as you work to remove surface starch and prevent browning.

Thinly slice the onion. Mince the garlic.

Drain the potatoes and pat dry thoroughly with clean kitchen towels or paper towels.

In a medium saucepan over medium heat, melt 2 tbsp unsalted butter. Add the minced garlic and cook 30–45 seconds until fragrant, without browning.

Stir in 1 tbsp all-purpose flour and cook, stirring, 1 minute to make a light roux.

Slowly whisk in 2 cups heavy cream and 1 cup whole milk, smoothing any lumps. Continue to whisk until the mixture is warm and begins to thicken, about 4–6 minutes.

Whisk in 1 tsp Dijon mustard, a pinch of freshly grated nutmeg, 1 tsp salt, and 1 tsp freshly ground black pepper. Taste and adjust seasoning; remember cheeses add salt.

Remove the sauce from heat once it coats the back of a spoon. Do not boil vigorously; keep it smooth and slightly thickened.

Arrange a single overlapping layer of potato slices in the bottom of the prepared baking dish, covering the surface but leaving a little room at the edges.

Scatter a few onion slices and a tablespoon of the grated Gruyère over the first layer.

Repeat layering potatoes, onions, and 1 cup of the Gruyère/Parmesan mixture (reserve about 1/2 cup Gruyère and 1/4 cup Parmesan for the top) until you've used most of the potatoes. Aim for about 3–4 layers depending on thickness.

Pour the warm cream sauce evenly over the assembled potatoes, letting it sink between layers. The sauce should come most of the way up the potato stack but need not fully cover the top.

Sprinkle the reserved Gruyère and Parmesan evenly over the top.

If using, mix 1/2 cup panko with 1 tbsp chopped fresh thyme and dot with small pieces of the remaining 1 tbsp butter; sprinkle this over the cheese for a crisp topping.

Cover the baking dish tightly with foil and bake in the preheated 375°F (190°C) oven for 40 minutes.

Remove foil and continue baking until the top is golden brown and liquid is bubbling around the edges, about 20–30 more minutes. If top is browning too quickly, tent lightly with foil.

To achieve an extra-deep golden crust, place under the broiler for 1–2 minutes, watching carefully to avoid burning.

Remove from oven and let the gratin rest 10–15 minutes so the cream sets and slices hold together.

Garnish with chopped chives or parsley and additional thyme if desired. Slice into portions and serve warm.