Crispy Truffle Potato Gratin with Gruyere Cream Step-by-step instructions

Before you start

Before you begin, set out all your ingredients and equipment. Heat the oven to 375°F (190°C) and position a rack in the center. Slice the potatoes as thinly as possible, ideally using a mandoline for uniform 1/8-inch rounds. Place the slices in a bowl of cold water to prevent browning while you work.

Thinly slice the leeks and rinse them well to remove any dirt. Grate the Gruyere and set it aside. In a small saucepan, combine the cream, garlic, thyme, salt, and pepper, and bring to a gentle simmer. Butter the skillet thoroughly so the gratin doesn't stick.

Instructions

Prep and Cream

1. 1Prepare the Potatoes and Leeks
   Heat the oven to 375°F (190°C). Slice the Yukon Gold potatoes into paper-thin rounds, about 1/8 inch thick, using a mandoline or a very sharp knife. Keep the slices in a bowl of cold water to prevent browning while you work. Thinly slice the leeks crosswise, using only the white and light green parts, and rinse them well to remove any grit. Pat the leeks dry with a clean kitchen towel.
   Look for Potato slices are uniformly thin and leeks are clean and dry.
   Tip A mandoline makes quick work of slicing and ensures even cooking.
2. 2Grate the Gruyere
   Grate the aged Swiss Gruyere on the large holes of a box grater into a fresh pile of shredded cheese. Set aside. You should have about 2 cups of loosely packed grated cheese.
   Look for Cheese is finely shredded and fluffy.
   Tip Grating your own cheese melts more smoothly than pre-shredded.
3. 3Make the Garlic Thyme Cream
   In a small saucepan, combine the heavy cream, minced garlic, fresh thyme leaves, kosher salt, and black pepper. Place over medium heat and bring to a gentle simmer, stirring occasionally. Let it bubble for 2 to 3 minutes until the cream is fragrant and slightly thickened. Remove from heat and set aside.
   Look for Cream is steaming and just beginning to bubble at the edges.
   Tip Do not let it boil hard or the cream may curdle.

At the end of this phase, you should have a neat pile of paper-thin potato slices, clean and dry leeks, a bowl of freshly grated Gruyere, and a saucepan of fragrant, bubbling garlic-thyme cream.

The cream should be steaming and just beginning to thicken, with the aroma of garlic and thyme filling the kitchen. Everything is ready to assemble.

Layer and Bake

4. 4Butter the Skillet
   Generously coat the bottom and sides of a 10-inch cast iron skillet with the softened unsalted butter. Make sure every surface is well covered to prevent sticking and to add flavor.
   Look for Skillet glistens evenly with butter.
   Tip Use a pastry brush or your fingers to spread the butter evenly.
5. 5Layer the Potatoes and Leeks
   Arrange the potato rounds in tight overlapping spirals in the buttered skillet, covering the bottom completely. Scatter a handful of the sliced leeks over the first layer. Repeat with another layer of potatoes, then leeks, until all potatoes and leeks are used, finishing with a neat layer of potatoes on top. Press down gently to compact the layers.
   Look for Potatoes form a neat, overlapping pattern with leeks tucked between.
   Tip Overlapping the slices helps the gratin hold together when sliced.
6. 6Pour the Cream Over
   Pour the bubbling garlic thyme cream evenly over the layered potatoes, making sure it seeps down through all the layers. Gently shake the skillet to help the cream settle. The cream should come about three-quarters of the way up the sides.
   Look for Cream is visible between the potato layers and nearly covers them.
   Tip If the cream doesn't reach the top, add a few tablespoons of milk or more cream.
7. 7Bake the Gratin
   Cover the skillet tightly with aluminum foil and bake in the preheated oven for 40 minutes. After 40 minutes, remove the foil and continue baking for 15 to 20 minutes more, until the top is golden brown and the potatoes are tender when pierced with a knife. The cream should be bubbling and slightly reduced.
   Look for Top is golden and crisp at the edges; a knife slides through potatoes easily.
   Tip If the top browns too quickly, cover loosely with foil again.

The skillet is buttered and filled with overlapping potato and leek layers, with the cream poured over. After baking, the top is golden brown and the potatoes are tender. The cream has reduced and thickened, and the edges are starting to crisp. The gratin should look bubbly and fragrant, with no raw potato visible.

Finish and Serve

8. 8Add the Gruyere and Broil
   Sprinkle the grated Gruyere evenly over the top of the gratin. Return the skillet to the oven and broil for 2 to 4 minutes, watching closely, until the cheese is melted, golden, and bubbling. The edges of the potatoes should be crispy and deeply browned.
   Look for Cheese is fully melted with browned spots and a crispy rim.
   Tip Broiling times vary; keep an eye on it to prevent burning.
9. 9Finish with Thyme and Truffle Oil
   Remove the skillet from the oven and let it rest for 10 minutes. Scatter fresh thyme leaves over the top and drizzle with truffle oil. The gratin will continue to set as it rests, making it easier to slice and serve.
   Look for Gratin is fragrant with thyme and truffle; the top is glossy.
   Tip Resting allows the layers to firm up slightly for cleaner portions.
10. 10Serve the Golden Wedges
    Using a spatula or a large spoon, lift a golden cheese-stretching wedge straight from the skillet. Pull a crispy cheese-stretching portion to serve. Spoon any extra cream from the skillet over the top. Serve hot, directly from the skillet for a rustic presentation.
    Look for Cheese pulls in long, stretchy strands; potatoes are tender and creamy.
    Tip Use a wide spatula to lift the wedge without breaking it.

The Gruyere has melted into a golden, bubbling crust, with some browned spots and crispy edges. Fresh thyme and truffle oil are scattered on top. After resting, the gratin is set enough to slice into clean wedges, and the cheese pulls into long, stretchy strands when lifted. The aroma is rich, nutty, and earthy.

Common mistakes

One common mistake is slicing the potatoes too thickly, which prevents them from cooking through evenly. Stick to 1/8 inch or thinner. Another is not drying the leeks thoroughly after rinsing, which can add excess water to the gratin and make it watery.

Be sure to pat them dry. Overfilling the skillet with too many potato layers can cause the top to burn before the center is done. Press the layers down gently to compact them. Finally, don't skip the resting time; cutting too soon will cause the gratin to fall apart.

How to tell it’s done

The gratin is done when the top is golden brown and the potatoes are tender when pierced with a knife. The cream should be bubbling and slightly reduced, and the edges of the potatoes should be crispy and browned. After adding the Gruyere and broiling, the cheese should be melted, golden, and bubbling.

Let the gratin rest for 10 minutes before serving; this allows the layers to set slightly, making it easier to slice. The internal temperature of the dish should be at least 165°F (74°C) if you want to be precise, but since there are no raw proteins, the visual cues are most important.