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

Before you start

Before starting, heat the oven and butter the skillet. Slice the potatoes as thinly as possible—using a mandoline makes this quick and consistent. Clean and slice the leeks, then grate the cheese. Have the cream, garlic, thyme, and seasonings measured and ready.

The cream mixture can be made while the potatoes are being sliced. The entire preparation takes about 20 minutes, and the baking time is roughly 50 minutes. Make sure the skillet is oven-safe and large enough to hold the layers comfortably.

Instructions

Preparation and Cream

1. 1Prep the Potatoes and Leeks
   Heat the oven to 375°F (190°C). Butter the bottom and sides of a 10-inch cast iron skillet generously. Using a mandoline or a sharp knife, slice the Yukon Gold potatoes into paper-thin rounds, about 1/8-inch thick. Clean the leeks thoroughly, then slice them thinly crosswise, using only the white and light green parts. Grate the aged Gruyere on the large holes of a box grater into a bowl.
   Look for Potato slices are thin and even; leeks are clean and dry; cheese is freshly grated.
   Tip Soak the sliced potatoes in cold water for 10 minutes, then pat dry to remove excess starch for a creamier texture.
2. 2Make the Garlic Thyme Cream
   In a small saucepan, combine the heavy cream, minced garlic, fresh thyme leaves, kosher salt, and black pepper. Warm over medium heat, stirring occasionally, until the mixture is steaming and small bubbles form around the edges. Do not let it boil. Remove from the heat and stir in the truffle oil, if using. Set aside.
   Look for The cream is fragrant with garlic and thyme, with tiny bubbles at the edges.
   Tip If the cream boils, the fat may separate; keep the heat gentle.

At the end of this phase, the potatoes are sliced thin and even, the leeks are clean and sliced, and the cheese is grated. The cream mixture is warm and fragrant, with garlic and thyme infused. The skillet is buttered and ready. You should have all the components prepped and within reach for easy assembly.

Layering and Baking

3. 3Layer the Gratin in the Skillet
   Arrange a single layer of potato rounds in the buttered skillet, overlapping them in a tight spiral. Scatter a handful of sliced leeks over the potatoes, then sprinkle with a portion of the grated Gruyere. Repeat the layers—potatoes, leeks, cheese—until all ingredients are used, finishing with a layer of potatoes. Pour the warm garlic thyme cream evenly over the top, pressing down gently with a spatula to ensure the cream reaches every layer.
   Look for The layers are compact and the cream just peeks through the top layer.
   Tip Overlap the potatoes in a circular pattern for even cooking and a pretty presentation.
4. 4Bake the Gratin
   Cover the skillet tightly with aluminum foil and place it in the preheated oven. Bake for 35 minutes, then 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 The top is golden and the edges are crispy; a knife slides in easily.
   Tip If the top browns too quickly, cover loosely with foil for the remaining time.

The layers are neatly arranged in the skillet, and the cream has been poured over. The gratin is baking, first covered, then uncovered. The kitchen smells of garlic, thyme, and melting cheese. The top is turning golden and the edges are sizzling. A knife inserted into the center meets tender potatoes.

Resting and Serving

5. 5Rest and Serve
   Remove the skillet from the oven and let the gratin rest for 10 minutes. This allows the cream to set slightly and makes serving easier. Scatter fresh thyme leaves over the top before serving. Use a spoon or spatula to lift out portions, pulling up the crispy cheese edges for maximum effect.
   Look for The gratin is set but still creamy, with crispy edges and a fragrant thyme garnish.
   Tip Letting it rest is key—if served immediately, it will be too loose.

The gratin has rested and is set but still creamy. The top is crispy and golden, with cheese pulling away from the edges. Fresh thyme adds a pop of color and fragrance. When lifted with a spoon, the layers hold together, and the cheese stretches invitingly. It's ready to serve.

Common mistakes

A common mistake is slicing the potatoes too thick, which results in a crunchy, undercooked gratin. Aim for even, paper-thin slices. Another issue is overfilling the skillet—leave a little room at the top so the cream doesn't bubble over.

Skipping the rest time can make the gratin runny and difficult to serve. Also, don't skip the foil cover; it helps the potatoes cook through without burning the top. Finally, be careful with the salt, as Gruyere is already salty.

How to tell it’s done

The gratin is done when the top is deep golden brown and the edges are bubbling and crispy. A knife or skewer inserted into the center should meet little resistance, indicating the potatoes are tender.

The internal temperature of the gratin should be at least 165°F (74°C) to ensure it's heated through, though this is not a protein dish.

The cream should be reduced and thickened, clinging to the potatoes rather than pooling. Letting it rest for 10 minutes allows the gratin to set and makes slicing easier.