1. Place chicken breasts in a single layer in a lightly greased 13- x 9-inch baking dish. Top evenly with onions, mushrooms, and tomatoes. Pour dressing evenly over chicken and vegetables.
2. Bake, covered, at 375° for 45 minutes. Remove chicken and vegetables from oven; let stand 10 minutes. Shred chicken using two forks.
3. Cook pasta according to package directions, and drain. Combine shredded chicken mixture and spaghetti, tossing well. Top evenly with 1/2 cup Parmesan cheese.
Note: For testing purposes only, we used Kraft Zesty Italian Dressing.