Bring a pot of heavily salted water to boil. Cook the pasta until very al dente (about 4 minutes shy of package directions) –it should have a definite bite to it. Drain and set aside.
In the meantime, cook the sausage in 1 tablespoon of olive oil over medium-high heat in a large skillet. Using your spatula, break up the pieces of meat as much as you can until fully browned. Set aside.
Using the fat left in the pan, sauté the fennel until translucent. Add the mushrooms and continue to cook until golden brown. Add the garlic and sauté for an additional two minutes. Cover the mixture with the heavy cream and simmer over high heat until reduced by half, about four minutes. Season to taste.
Toss the pasta and sausage together with the cream mixture, pecorino, and 1/3 of the mozzarella cheese. Distribute evenly in a casserole, gratin, or baking dish and cover with the remaining cheese.
Bake uncovered until golden brown on the top, about 5 to 7 minutes.
Phoebe is a writer, gluten-free chef, culinary instructor, award-winning blogger, and author of The Wellness Project. You can find her healthy comfort food and gluten-free finds on www.FeedMePhoebe.com