Leek and Potato Gratin

0 Preheat the oven to 350 degrees F.
1 Melt the butter in a large skillet or sauté pan over medium-low heat. Once melted, add the leeks, garlic and thyme and season with salt and pepper. Cook by stirring occasionally and until the leeks are soft about 8 to 10 minutes. 
2 Add the cream and bring to a boil. Season with salt and pepper, reduce the heat to a simmer, and let the leeks and cream cook for another 4 minutes.
3 Remove from the heat and add the thin potato slices to the leeks. Toss gently to combine thoroughly.
4 In a gratin dish, layer half of the potato-leek mixture and sprinkle with half of the gruyere cheese. Cover with the remaining half of the potatoes and top with the remaining gruyere cheese.
5 Bake until the potatoes are very tender, and the cheese has melted and turned a golden brown, about 1 hour.
6 Let the gratin rest for about 5 minutes before serving.