Au Gratin Potatoes Recipe

Preheat and slice

Preheat the oven to 375°F (190°C). Butter a 9x9-inch or similar-sized baking dish with 1 tablespoon of the unsalted butter. Using a mandoline or a sharp knife, slice the peeled potatoes evenly to about 1/8-inch (3 mm) thick. Rinse the slices briefly under cold water and pat dry with a clean kitchen towel or paper towels to remove excess surface starch.

Arrange an even layer of potato slices in the prepared dish, overlapping slightly. Season this first layer with about one-third of the salt and pepper and scatter a few of the sliced onions and a little minced garlic over the potatoes. Continue layering potatoes, seasoning, onions and garlic until all are used, finishing with a layer of potatoes on top.

Warm cream and infuse

In a medium saucepan, combine the whole milk and heavy cream. Warm over medium heat until the mixture is hot but not boiling (small bubbles at the edge), about 5 minutes. Stir in the Dijon mustard, the freshly grated nutmeg (if using), and the remaining 1 teaspoon salt and 1/2 teaspoon pepper. Taste carefully and adjust seasoning — the potatoes will absorb some salt.

Add cheese

Remove the saucepan from the heat. Stir in 1 cup of the grated Gruyère and all the grated Parmesan until melted and smooth. Reserve the remaining 1/2 cup Gruyère for the top layer.

Combine and bake

Pour the hot cream-cheese sauce evenly over the layered potatoes, using a spatula to nudge the sauce between slices so it distributes. Press down lightly to compact the layers. Scatter the reserved grated Gruyère over the top. Sprinkle the breadcrumbs evenly if using, and dot the remaining 1 tablespoon butter over the top in small pieces to help browning. Cover the dish tightly with aluminum foil.

Bake covered in the preheated oven for 35 minutes. Remove the foil and continue baking until the top is golden and the potatoes are tender when pierced with a knife, about 20–25 more minutes (total baking time 55–60 minutes). If you prefer a deeper brown crust, place under the broiler for 1–2 minutes, watching closely.

Remove the gratin from the oven and let it rest 10–15 minutes; this helps the sauce set so the pieces hold together when serving. Sprinkle the fresh thyme leaves (or parsley) over the top before serving.