Potato-Leek Gratin

INGREDIENTS

• 5 Tbs. unsalted butter
• 4 lb. leeks, white and light green portions, rinsed well, cut into 1/4-inch rings
• 1 1/2 Tbs. kosher salt
• 1 tsp. minced fresh thyme
• 1/4 tsp. freshly grated nutmeg
• 3/4 tsp. freshly ground pepper
• 1 cup heavy cream
• 6 oz. Gruyère cheese, grated
• 3 oz. Parmigiano-Reggiano cheese, grated
• 3 lb. russet potatoes, peeled, cut into 1/4-inch slices
• 3 Tbs. minced fresh chives