1. Heat oven to 375 degrees. Spray 9 ½ in glass deep-dish pie plate with cooking spray.
2. Cook and drain pasta as directed on package. In a large bowl, mix together butter, milk, flour. Stir in cooked pasta, tomato and cheeses. Gently fold in tuna. Spoon into the pie plate.
3. Cover with foil and bake for 30 minutes. Uncover; stir and sprinkle with onions evenly over top. Bake 5 minutes longer or until onions are light golden brown. Serve warm.