Pommes Dauphinoise

• 2 tablespoons unsalted butter, divided
• 1 leek, white and tender green parts only, halved lengthwise and thinly sliced
• 1 teaspoon anchovy paste or 1 1/2 anchovy fillets, very finely chopped
• 2 cups heavy cream
• 1 cup whole milk
• 1 garlic clove, finely chopped
• 2 fresh thyme sprigs
• 1/4 teaspoon freshly grated nutmeg
• 2 1/2 pounds Yukon Gold potatoes
• Kosher salt
• Freshly ground black pepper
• 1 small red onion, halved and thinly sliced
• 6 ounces freshly shredded Gruyère
• Fresh thyme leaves for garnish

Preheat the oven to 400°F. Use 1 tablespoon of the butter to grease a 2-quart gratin dish.

In a sauté pan over medium heat, melt the remaining 1 tablespoon of butter. Stir in the leeks and cook until tender, stirring occasionally, about 5 minutes. Stir in the anchovy paste and warm through, about 30 seconds. Remove the pan from the heat and set it aside.

In a small saucepan over medium-low heat, stir together the cream, milk, garlic, thyme sprigs, and nutmeg. Bring to a simmer and keep warm over very low heat.

Peel the potatoes. Use a mandoline or other adjustable-blade vegetable slicer to cut them into thin, uniform rounds about 1/16-inch-thick. Arrange half of the potato slices in the bottom of the prepared dish. Sprinkle with salt, pepper, half of the red onions, half of the leek mixture, and half of the Gruyère. Make a second layer in the same way.

Discard the thyme sprigs from the cream mixture, then pour over the potatoes, letting it trickle down between the layers.

Bake until bubbling and golden brown on top, 45 to 55 minutes. Let stand 10 minutes, garnish with thyme leaves, and serve warm.