Baked Fettucheesy

Baked Fettucheesy
1 (16 ounce) package dry fettuccine pasta
1 tablespoon butter
1/2 onion, finely chopped
1 cup heavy cream
1/2 cup grated Parmesan cheese
1/2 cup grated Romano cheese
salt and freshly ground black pepper to taste
1 cup shredded mozzarella cheese
Preheat oven to 350 degrees F (175 degrees C). Lightly grease a medium casserole dish.  Bring a large pot of lightly salted water to a boil. Add pasta, and cook for 8 to 10 minutes or until al dente; drain.  In a skillet over medium heat, melt the butter, and cook the onion until tender. Stir in the fettuccine, heavy cream, 1/4 cup Parmesan cheese, 1/4 cup Romano cheese, salt and pepper. Transfer to the prepared dish. In a bowl, mix the remaining Parmesan, remaining Romano, and mozzarella. Sprinkle the cheese mixture over the fettuccine mixture.  Bake 15 minutes in the preheated oven, until bubbly and golden brown.