1. Position a rack in the center of the oven and preheat the oven to 350°F. Lightly butter a 3-quart shallow baking dish.

2. Heat the oil in a very large skillet over medium-high heat. Add the mushrooms and cook, stirring occasionally, until the mushrooms are lightly browned, about 10 minutes. Season with salt and pepper. Set aside.

3. Meanwhile, bring a large pot of lightly salted water to a boil over high heat. Add the pasta and cook according to the package instructions until it is not quite al dente (it will cook more in the oven). Drain.

4. Return the pasta pot to medium heat. Add 4 tablespoons of the butter and melt. Whisk in the flour and reduce the heat to low. Let the roux bubble without browning for 1 minute. Whisk in the hot milk and bring to a boil over high heat. Stir in the Gruyere. Remove from the heat. Stir in the pasta and mushrooms, and season with salt and pepper. Spread in the baking dish. Sprinkle with the bread crumbs. Cut the remaining 1 tablespoon butter into small cubes and dot over the bread crumbs. Bake until the topping is brown and the sauce is bubbling, about 25 minutes. Serve hot.