COOK spaghetti until firm but not soft (al dente); DRAIN; PLACE in a greased 11-inx 7-inx 2-inbaking dish; TOP with chicken; SET aside.
In a large skillet, SAUTE mushrooms and onion in butter until tender.
WHISK in the soup, milk, and mustard until blended.
STIR in the cheddar cheese; COOK, STIRRING frequently, over medium heat until cheese is melted.
POUR mixture over chicken; SPRINKLE with Parmesan and mozzarella cheeses.
BAKE, uncovered, at 350° until heated through and cheese is melted, about 25-30 minutes.
May substitute turkey for chicken
Per Serving (excluding unknown items): 803 Calories; 41g Fat (46.0% calories from fat); 54g Protein; 53g Carbohydrate; 2g Dietary Fiber; 171mg Cholesterol; 1004mg Sodium. Exchanges: 3 Grain(Starch); 6 Lean Meat; 1/2 Vegetable; 0 Non-Fat Milk; 5 Fat.