Potato-leek gratin

1. In a 12-inch nonstick fry pan over medium heat, melt 4 tbsp of the butter. Add the leeks and salt, stirring to coat the leeks with the butter. Cover and cook, stirring occasionally, until the leeks are tender, about 20 minutes. Add the thyme, nutmeg, pepper and cream and simmer, uncovered, stirring occasionally, until thickened, about 15 minutes. Transfer to a bowl and let cool. Wipe out the pan and grease with the remaining 1 tbsp butter. Preheat oven to 400. IN a bowl, combine the Gruyere and Parmigiano-Reggiano cheeses. Layer one-third of the potatoes in the fry pan and spread one-third of the leek mixture on top. Sprinkle with one-third of the cheeses, then 1 tbsp of the chives. Repeat the layering 2 more times, reserving 1 tbsp of the chives. Cover the pan, transfer to the oven and bake for 45 minutes. Remove the lid and continue baking until the potatoes are tender and the crust is golden brown, about 30 minutes more. Sprinkle the remaining 1 tbsp of chives on top. Let the gratin stand for 15 minutes before serving