Au Gratin Potato Recipe

Prep and parboil

Preheat oven to 375°F (190°C). Peel potatoes if you prefer (Yukon Gold can be left unpeeled); slice potatoes into 1/8- to 1/4-inch (3–6 mm) rounds using a sharp knife or mandoline for even thickness. Place slices in cold water while you prepare the rest to prevent discoloration.

Bring a large pot of water to a boil, add 1 tablespoon kosher salt. Add potato slices and simmer 5 minutes — they should be slightly tender but still hold shape (parboiled). Drain in a colander and let steam-dry for 2–3 minutes.

Butter a 9x9-inch (or similar 2–3 quart) baking dish with 1 tablespoon unsalted butter; spread to coat the bottom and sides.

Make the sauce

In a medium saucepan, melt 1 tablespoon unsalted butter over medium heat. Add minced garlic and cook 30–45 seconds until fragrant but not brown. Sprinkle 1 tablespoon flour over the butter/garlic and whisk for 30–45 seconds to cook the flour (roux).

Gradually whisk in 1 cup whole milk and 1 cup heavy cream, smoothing lumps as you go. Increase heat to medium-high and bring mixture to a gentle simmer, whisking constantly. The sauce should thicken to coat the back of a spoon in 4–6 minutes.

Stir in 1 teaspoon Dijon mustard, 1 teaspoon salt, 1 teaspoon freshly ground black pepper, and 1 teaspoon freshly grated nutmeg. Taste and adjust seasoning. Remove from heat and set aside.

Combine cheeses

In a medium bowl, toss together 1 cup grated Gruyère, 1 cup grated sharp cheddar, and 1 cup finely grated Parmesan until well combined. Reserve 2 tablespoons of the Parmesan for sprinkling on top later.

Arrange a single layer of parboiled potato slices in the bottom of the prepared dish, slightly overlapping. Sprinkle about one-third of the cheese mixture evenly over this layer (use approximately 1/3 of the total cheese).

Ladle enough warm cream sauce to moisten the layer — roughly 1/3 of the sauce — so the cheese begins to melt but not so much that the dish becomes soupy. Continue layering potatoes, cheese (one-third each layer), and sauce until you have 3 layers or run out of potatoes, finishing with a cheese layer. Press down gently to compact layers and ensure even distribution of sauce.

Top and bake

Sprinkle reserved 2 tablespoons Parmesan evenly over the top cheese layer. Cover the dish tightly with foil and bake in preheated 375°F (190°C) oven for 25 minutes. Remove foil and continue to bake for an additional 20–25 minutes until potatoes are tender when pierced with a knife and top is golden and bubbling.

If you want a more deeply browned crust, switch oven to broil for 2–3 minutes while watching closely to prevent burning.

Remove from oven and let rest 10 minutes to set. While resting, sprinkle crumbled cooked bacon (6 slices cooked crisp and crumbled) evenly over the top and scatter 2 tablespoons sliced fresh chives. Serve warm.