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 brow.