Cheesy Pasta Bake

1 pound mixed wild mushrooms, such as oyster, maitake and shiitake
3 tablespoons extra-virgin olive oil 
1/2 teaspoon kosher salt, more as needed
1/2  teaspoon black pepper, plus a few grinds
2 fresh rosemary branches
1/2 pound orecchiette, farfalle or other short pasta
3/4 cup heavy cream
1/2 cup fresh ricotta
5 ounces fontina cheese, grated (1 1/4 cups)
2 ounces Parmesan, grated (1/2 cup)
1 teaspoon finely chopped fresh sage
1 garlic clove, finely grated
(1 hot italian sausage, crumbled, cooked)

PREPARATION
Heat the oven to 450 degrees. Trim the mushrooms and cut into 1-inch pieces. Toss with the olive oil, the salt, a few grinds of pepper and the rosemary. Spread on a large baking sheet and roast, tossing once or twice, until golden brown and crisped around the edges, 15 to 18 minutes.

Meanwhile, bring a large pot of salted water to a boil. Add the pasta and cook for at least a few minutes less than the package directs. (You want the pasta very al dente; it will finish softening in the sauce.) Drain well.

Turn oven up to 500 degrees. In a large bowl, stir together the cream, ricotta, fontina, Parmesan, sage, pepper, garlic and a pinch of salt. Stir in the pasta and mushrooms. Arrange in a shallow 2-quart gratin dish or 9- by 13-inch pan. Bake until cheese is melted and bubbly and browned in spots, 10 to 15 minutes.