Creme Fraiche Potato Gratin

3 leeks1 ounce cultured butter (or regular butter)2 cloves garlic, peeled and mincedSalt and freshly ground black pepper, to taste1 tablespoon fresh thyme, chopped, or 11/2 teaspoons dried thyme1#189; cups whole milk8 ounces creme fraiche4 pounds Yukon gold potatoes, peeled and sliced thin1 cup cheddar or Gruyere cheese, gratedHeat the oven to 400 degrees. Butter an ovenproof 10-by-14-inch gratin or casserole dish, and set aside. Trim the roots off the leeks, and slice off and discard most of the stems, leaving the white part and just a bit of the green. Slice in half lengthwise, and rinse well under cold running water, making sure to remove any dirt. Drain well. Chop.Melt the butter in a large skillet set over medium heat. Add the leeks and garlic, stirring occasionally, and cook until the leeks become somewhat translucent and caramelized. Add the salt, pepper, thyme and milk and, stirring constantly, allow to come to a boil. Lower heat, and stir in the creme fraiche. Add the sliced potatoes, and allow to simmer for a few minutes. Make sure to gently stir the potatoes so all are coated. Spoon the potatoes and cream sauce into the prepared dish, scraping the skillet to get every bit. Sprinkle with the cheese. Bake 1 hour, until the potatoes are soft and the cheese is browned and bubbling.Serves six.Chef Dan Barber, Blue Hill Restaurant, New York, N.Y., from "In a Cheesemaker's Kitchen" by Allison Hooper (2009 Countryman Press)