Prepare rigatoni according to pkg directions. Spread oil on bottom of 11×7 in baking dish; add onion in a single layer. Bake at 375 for 15 minutes or just until tender. Transfer onion to large bowl, set aside.
Drain chopped spinach well, pressing between paper towels. Stir in rigatoni, spinach, chicken, & next 4 ingredients into onion in bowl. Spoon mixture into dish & sprinkle evenly with shredded mozzarella cheese. Bake covered at 375 for 30 minutes; uncover & bake 15 more minutes or until bubbly.