Mushroom and potato gratin with thyme and parmesan

First made for Christmas 2008. We made it with morel, porcini, chantrelle, and baby bella mushrooms. Also, in step 5, we only poured about half the cream through the strainer into the dish - too heavy otherwise! A Williams-Sonoma recipe.

Ingredients

 * 1 Tbs. unsalted butter, plus more for greasing
 * 1 1/2 cups heavy cream
 * 1 garlic clove, thinly sliced
 * 3 fresh thyme sprigs, plus 1 1/2 tsp. minced
 * Sea salt and freshly ground pepper, to taste
 * 2 lb. Yukon Gold potatoes
 * 1 Tbs. extra-virgin olive oil
 * 1 lb. mixed wild and cultivated mushrooms, woody stems removed, caps thinly sliced
 * 4 Tbs. grated Parmigiano-Reggiano cheese

Preparation

 * 1) Preheat an oven to 375°F. Butter an 8-inch square baking dish.
 * 2) In a saucepan, combine the cream, garlic, thyme sprigs, and a pinch each of salt and pepper. Bring to a low boil over medium heat, then remove from the heat and set aside.
 * 3) Peel the potatoes and, using a mandoline or a very sharp knife, cut them into slices about 1/8 inch thick. Gently stir the potato slices into the cream mixture, cover and let stand while you cook the mushrooms.
 * 4) In a fry pan over medium heat, warm the olive oil and melt the 1 Tbs. butter. Add the mushrooms and a pinch of salt, and sauté until all of the liquid released by the mushrooms has evaporated, 7 to 9 minutes. Add the minced thyme with a pinch of pepper and cook for 1 minute.
 * 5) Arrange one-third of the potato slices, slightly overlapping, on the bottom of the prepared dish. Sprinkle lightly with salt and pepper and 1 Tbs. of the cheese. Spread half of the mushrooms over the potatoes and sprinkle with 1 Tbs. of the cheese. Repeat these layers, using half of the remaining potatoes and all of the remaining mushrooms, and sprinkling with salt, pepper and 1 Tbs. cheese between the layers of vegetables. Top with the remaining potatoes and sprinkle with salt and pepper. Using a large, flat spatula, gently press on the vegetables to compact them. Pour the cream mixture through a strainer into the dish and sprinkle with the remaining cheese.
 * 6) Cover the dish with aluminium foil and bake until the potatoes are tender when pierced with a sharp knife, about 45 minutes. Remove the foil and bake until the mixture is golden brown and bubbly, about 20 minutes more. Let the gratin rest for about 10 minutes, then use a sharp-bladed spatula to cut it into squares and serve immediately. Serves 4 to 6.