Cut sausage in half lengthwise, then cut in slices.
Cook pasta in salted water till al dente. Add spinach right at the end so it will wilt. Drain and set aside.
Meanwhile, in a large skillet, cook sausage and onion in oil over medium heat till onion is tender, about 5 minutes. Stir in garlic and cook for another minute.
Stir in tomatoes, oregano and basil; simmer for 8-10 minutes.
Add cream, cubed mozzarella, pasta, and spinach. Heat through.
Pour mixture into a greased 9x13" pan. Top with parmesan and grated mozzarella cheese.
Bake at 400 degrees for 20-30 minutes, or until edges are bubbly and cheese is lightly browned.