Heat oven to 375°F. Spray 9 1/2-inch glass deep-dish pie plate with cooking spray.
Cook and drain pasta as directed on package. In large bowl, mix soup and milk. Stir in cooked pasta, tomato and cheeses. Gently fold in tuna. Spoon into pie plate.
Cover with foil; bake 30 minutes. Uncover; sprinkle onions evenly over top. Bake 5 minutes longer or until onions are light golden brown.