Tomato, Onion and Potato Gratin

Recipe from Chez Panisse Vegetables

(serves 6-8)
1 medium yellow onion, as thinly sliced as possible
2 1/2 lbs. red or yellow potatoes
1 1/2 lbs. tomatoes, sliced 1/8-inch thick
2 cloves garlic, thinly sliced
salt and pepper
1 Tablespoon chopped thyme leaves
3 Tablespoons extra virgin olive oil
2 Tablespoons white wine
1 1/2 cups chicken stock

You will need a 9-inch square or oval gratin dish with 2-inch sides. Preheat the oven to 375. Peel and slice potatoes 1/8 inch thick and hold in cold water until ready to use. Lightly oil the dish and distribute the onion slices evenly on the bottom. Drain the potatoes and arrange half of them over the onions, overlapping like shingles. Season with salt, pepper and thyme. Make a layer of tomato slices on top of the potatoes, salt and pepper them and layer with remaining thyme and garlic slices. Finish with a final layer of potatoes, more salt and pepper. Drizzle with olive oil and white wine, and add enough chicken stock to come 2/3 inch up the sides. Cover with foil and bake for 40 minutes. Uncover and press down on the gratin, flattening to ensure the top layer of potatoes is moistened. Continue to bake uncovered for 40 minutes until a knife pierces easily and the top is golden-brown. Serve warm or at room temperature.