- 5 ounces crumbled blue cheese (such as Stilton, Gorgonzola or Maytag blue; about 1 1 /2 cups), room temperature
- 2 1/2 cups whipping cream
- 1 teaspoon salt
- 1/2 teaspoon pepper
- 1 1/2 tablespoons butter
- 1 pound mixed fresh wild mushrooms (such as chanterelle, morel, portobello, stemmed shiitake, oyster, crimini or hedgehog), thickly sliced
- 1 1/2 teaspoons chopped fresh thyme or 3/4 teaspoon dried
- 2 pounds Yukon Gold potatoes, peeled, very thinly sliced into rounds
- Position rack in top third of oven and preheat to 400°F. Butter 13 x 9 x 2-inch glass baking dish. Place cheese in medium bowl; add 1 cup/2 cream. Using fork, mash mixture to chunky paste. Mix in 1 teaspoon salt and 1/2 teaspoon pepper. Mix in remaining 2 cups cream.
- Melt butter in heavy large pot over medium-high heat. Add mushrooms and herbs; sauté until mushrooms are tender and liquid cooks away, about 8 minutes. (Cheese sauce and mushrooms can be made 8 hours ahead. Cover separately and chill.)
- Arrange half of potatoes on bottom of prepared dish. Spoon 3/4 cup cheese sauce evenly over. Top with all of mushroom mixture, 3/4 cup cheese sauce, then remaining potatoes. Top with remaining cheese sauce. Cover dish with foil.
- Bake gratin 30 minutes. Uncover and bake until potatoes are tender, top is golden brown and sauce is thickened, about 30 minutes longer. Let stand 10 minutes; serve hot.