Potato, mushroom, and gruyere gratin
Instructions:
- Position a rack in the middle of the oven and preheat oven to 400 F. Butter the bottom of a shallow 2- or 21⁄2-quart baking dish.
- Melt 1 tablespoon of the butter in a large skillet over medium-high heat. Add the wild mushrooms and cook, stirring often, until they soften and any liquid evaporates, about 8 minutes. Work in batches, if necessary, to avoid crowding the mushrooms in the pan so that they can brown well and stay firm instead of getting slimy and lost in lots of liquid. Pour the cooked mushrooms onto a plate and set aside.
- Melt the remaining 1 tablespoon of butter in the skillet and cook the cremini mushrooms the same way. Transfer them to the plate and set aside.
- Stir together the cream, milk, garlic, salt, pepper, thyme, rosemary, and nutmeg in a large saucepan.
- Working with one potato at a time, peel the potatoes and cut into even slices that are no more than 1⁄8-inch thick and stir them into the cream mixture. A vegetable slicer is ideal, although the slicing can be done with a sharp knife. To make the process easier and safer when using a knife, cut a thin slice off one of the long sides of the potato so that it sits flat and does not wobble on the cutting board.
- Bring the potatoes just to a simmer over medium-high heat, then remove the pan from the heat. Use a slotted spoon to transfer half of the potatoes to the prepared dish, spreading them evenly. Top with the cooked mushrooms but leave any accumulated mushroom liquid behind. Spoon the remaining potatoes over the mushrooms, spreading evenly. Pour the potato cooking liquid evenly over the top and sprinkle with the cheese.
- Bake until the top is browned and the potatoes are tender and bubbling around the edges, 45 to 55 minutes. If the top gets too brown before the potatoes are done, lay a flat sheet of aluminum foil over the top. Let stand at least 10 minutes before serving.
- What else works? You can replace up to 8 ounces of the potatoes with thinly sliced celery root or Jerusalem artichokes. Another nice variation is toomit the mushrooms and increase the total amount of potatoes to 2 pounds, using a combination of russets and sweet potatoes.
- Tips and Techniques. There are four simple things that make a potato gratin thick and creamy instead of runny. First, use very starchy potatoes, such as russets. Second, slice the potatoes directly into the cream so that they don’t oxidize and turn the color of wet newspaper before they’re cooked. Third, heat the potatoes in the cream before layering them in the dish. This gives the potatoes a chance to release some of their starch into the cream, which helps thicken the liquid. Fourth, let the gratin rest for at least 10 minutes before serving to give it time to firm up.