You can substitute other varieties of mushrooms, such as cremini or white, for the portobellos in this savory baked dish.
Bring a large pot of water to a boil. Add pasta; cook according to package instructions until al dente. Drain; set aside. Pulse bread in a food processor until coarse crumbs form (you should have 1 cup plus 2 tablespoons); set aside.
Preheat oven to 375 degrees. Whisk milk, flour, chopped sage, salt, and pepper in a medium saucepan. Bring to a boil, whisking often. Add 3 ounces cheese, and cook, whisking constantly, until thick enough that whisk leaves a trail, about 2 minutes more. Remove from heat, and set aside.
Lightly coat six 10-ounce gratin dishes with cooking spray. Set on a rimmed baking sheet, and space at least 1 inch apart; set aside. Toss mushrooms, pasta, and cheese mixture in a large bowl. Divide among prepared dishes. Crumble the remaining cheese over each, dividing evenly, and sprinkle with breadcrumbs.
Cover each dish with foil. Bake until mushrooms are tender, 25 to 30 minutes. Remove foil. Bake until the breadcrumbs are browned, about 7 minutes more. Transfer to wire racks; let cool 5 minutes. Garnish each gratin with 2 sage leaves.