Potato, Mushroom, and Roquefort Gratin

2 tablespoons unsalted butter
1 pound shiitake mushrooms, stemmed and cut into ½- inch-thick slices
1/3 cup chopped shallots
2 cups heavy cream
1 cup Homemade White Chicken Stock or canned reduced-sodium chicken broth
6 ounces Roquefort or other blue cheese, crumbled
5 large (about 3¼ pounds) baking potatoes, such as russet or Burbank, peeled and thinly sliced
½ teaspoon salt
¼ teaspoon freshly ground black pepper

A gratin is the French way of making scalloped potatoes.

Position a rack in the top third of the oven and preheat to 350°F. Lightly butter a 13 × 9-inch baking dish.
In a large skillet, heat the butter over medium heat. Add the mushrooms and cook, stirring often, until tender, about 10 minutes. Stir in the shallots and cook until softened, about 1 minute. Set aside.
In a medium saucepan, bring the cream and stock just to a simmer over medium heat. Remove from the heat and stir in the cheese until melted.
Spread half of the potatoes over the prepared dish and sprinkle with half of the salt and half of the pepper. Top with the mushrooms, followed by the remaining potatoes. Season with the remaining salt and pepper. Pour the hot cream mixture over all. Cover tightly with foil.
Bake for 40 minutes. Remove the foil and continue baking until the potatoes are tender and the top is golden, about 40 minutes more. (The gratin can be made up to 2 hours ahead of serving. Let stand at room temperature. Cover loosely with aluminum foil and reheat in a preheated 350°F oven until heated through, about 20 minutes. Or cover with plastic wrap and reheat in a microwave on Medium for about 10 minutes.) Let stand for 10 minutes before serving.