Potato and Leek Gratin

1 1/2 lbs. russet potatoes
2 tbsp. butter
1 leek, white part only, halved lengthwise and cut into 1/4-inch-thick slices
3/4 c. heavy cream
1/2 c. milk
1/8 tsp. nutmeg
Salt and pepper
1 c. Gruyere cheese, grated

1. Preheat oven to 400 F.
2. Fill large bowl with water. Peel potatoes and add them to water to prevent discoloration.
3. Melt butter in medium saucepan over medium heat. Add leeks and cook, stirring, until soft (3-5 min). Add cream, milk and nutmeg; cook until bubbles form around edge. Remove from heat.
4. Meanwhile, slice potatoes 1/8-inch thick. Place in even layers in a large shallow baking dish; season with salt and pepper. Pour leek-cream mixture over potatoes; sprinkle with cheese.
5. Transfer to oven and bake until potatoes are fork-tender and top is brown and bubbling, about 55 min. (45 min. covered; 10 min. uncovered). Remove from oven and let cool for 10 min. before serving.