Potato Gratin with Leeks & Fontina

Preheat the oven to 400°. In a large skillet, melt the butter over medium heat. Add the leeks. Cook, stirring often, until softened, 4 to 5 minutes. Stir in the cream and sage. Bring to a gentle boil. Add 1 cup cheese and stir until melted, a minute or two. Stir in 2 tsp. salt and 1 tsp. pepper. Remove from heat. Add the potatoes and stir to evenly coat the slices. Transfer to a 9-by-13-inch baking dish, spreading the potatoes in an even layer. Top with the remaining 1 cup cheese.

Cover the dish with foil. Bake for 30 minutes. Uncover the gratin and bake until the top is golden brown in spots and the potatoes are tender when pierced with a knife, 25 to 30 minutes more. Let the gratin rest for 15 minutes before serving.