Preheat oven to 375°F. Lightly spray bottom of 9x13" baking dish. Drain chopped spinach well, pressing between paper towels.
In a large mixing bowl, add drained spinach, sautéed onions, cooked rigatoni, cooked chicken, diced tomatoes, cream cheese, salt and pepper; mix together thoroughly. Spoon mixture into baking dish. Sprinkle shredded mozzarella cheese evenly over top.
Cover and bake for 30 minutes; uncover and continue baking 15 minutes longer or until bubbly.
If you do not have cooked chicken and onions in the freezer, you can sauté those while you are cooking the pasta.
For a vegetarian option, omit the chicken and add sautéed mushrooms and diced red peppers or other vegetables.