Prepare rigatoni according to package directions.
Meanwhile, spread oil on bottom of an 11- x 7-inch baking dish; add onion in a single layer.
Bake at 375° for 15 minutes or just until tender. Transfer onion to a large bowl, and set aside.
Drain chopped spinach well, pressing between layers of paper towels.
Stir rigatoni, spinach, sausage, and next 3 ingredients into onion in bowl. Spoon mixture into baking dish, and sprinkle evenly with shredded mozzarella cheese.
Bake, covered, at 375° for 30 minutes; uncover and bake 15 more minutes or until bubbly.
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