Cook spaghetti in a large pot of boiling, salted water until al dente.
In a large bowl, mix together cooked spaghetti, chopped chicken, cream of mushroom soup, canned mushrooms, onion, broccoli and carrots, and diced tomatoes. Pour mixture into a greased 2 quart casserole dish. Place cubed processed cheese food on top of dish.
Bake in a preheated 350 degree F (175 degrees C) for 30 minutes, or until cheese is bubbly and melted.
