2 tablespoons unsalted butter, plus more for buttering the baking dish
1 medium leek, white part only, halved lengthwise and cut crosswise into 1/4-inch slices
1 medium garlic clove, minced
1 teaspoon minced thyme leaves
2 1/2 pounds Yukon Gold potatoes, peeled and sliced about 1/8 inch thick
1 cup grated Gruyere cheese
Steps
Preheat oven to 350°F.
In a small saucepan, combine milk, cream, bay leaf, and nutmeg. Heat over medium until simmering. Remove from heat and set aside to infuse, then season with salt and pepper.
In a medium skillet, melt butter over medium heat. Add leeks and sauté until tender, about 5 minutes. Add garlic and cook for 1 minute until fragrant. Stir in thyme and cream mixture; discard bay leaf.
Generously butter a 9” x 13” gratin dish. Layer potatoes in even layers, seasoning each layer with salt and pepper. Pour leek-cream mixture over potatoes and sprinkle with Gruyere.
Bake until potatoes are fork-tender and cheese is golden brown, about 50 minutes. Let cool slightly before serving.