Cook pasta according to package directions. Meanwhile, in a Dutch oven, cook sausage and onion over medium heat until meat is no longer pink; drain., Stir in the tomatoes, spinach, cream cheese and 1 cup mozzarella cheese. Drain pasta; add to sausage mixture. Transfer to a greased 11x7-in. baking dish., Cover and bake at 350° for 35 minutes. Uncover; sprinkle with remaining cheese. Bake 10 minutes longer or until cheese is melted.