Ingredients

1. 2 pounds Yukon gold potatoes, peeled and sliced lengthwise 1/8 inch thick
2. 2 medium onions, thinly sliced
3. 2 garlic cloves, smashed
4. Salt and freshly ground pepper
5. 1/4 cup chicken fat, duck fat or olive oil (see Note)
6. 1 cup dry red wine
7. 1/2 cup leftover pan juices or veal, beef or chicken demiglace (see Note)
8. 1 large thyme sprig
9. 1 bay leaf
10. 1/4 cup water

Directions

1. Preheat the oven to 375°. Lightly grease a 3-quart shallow baking dish. In a large bowl, toss the sliced potatoes with the onions and garlic and season with salt and pepper. Heat 2 tablespoons of the fat in each of 2 large skillets. Divide the potatoes between the skillets and cook over moderate heat, turning the potatoes with a spatula, until browned in spots, about 10 minutes. Scrape the potatoes from 1 skillet into the other, add the wine and pan juices to the potatoes and bring to a boil.
2. Transfer the potatoes and liquid to the prepared baking dish and smooth the surface. Top with the thyme and bay leaf and bake for 30 minutes. Moisten the potatoes with the water and bake for about 25 minutes longer, or until the potatoes are tender and lightly browned. Serve the potato gratin hot.

Make Ahead

The potatoes can be prepared 4 hours ahead. Cover and reheat in a 325° oven.

Notes

When Lydie doesn't have any fat or pan juices in her refrigerator, she brings a jar to her butcher, who roasts chickens on a spit, and he fills it with fat and juices. If you're not a regular customer, you might have to buy a chicken, too, the first time.