Preheat oven to 350 degrees. Coat a 9-inch square pan with cooking spray.
ook pasta, al dente (not overcooked). In the meantime, place onions and celery on a microwave-safe plate or bowl. Cook in microwave for 2-3 minutes, to soften.
In a medium bowl, add drained tuna, Alfredo sauce, softened onions/celery, and mix well. Stir in cooked and drained pasta. Mix in 2 tablespoons Parmesan cheese. Blend well.
Pour mixture into baking dish. Sprinkle remaining 2 tablespoons of cheese evenly over the top. Bake for 30 minutes, uncovered, until golden brown.
Let cool for at least 10 minutes, to set. Cut into 6 servings.