Stir 1 cup stock, garlic powder, tomatoes, mushrooms and onions in a 5- to 6-quart slow cooker. Add the chicken and turn to coat.
Cover and cook on LOW for 7 to 8 hours* or until the chicken is cooked through. Remove the chicken from the cooker and keep warm.
Stir the cornstarch and remaining stock in a small bowl until the mixture is smooth. Stir the cornstarch mixture in the cooker. Cover and cook on HIGH for 10 minutes or until the mixture boils and thickens. Serve with the chicken and spaghetti.
*Or on HIGH for 4 to 5 hours.
Serving Suggestion: Serve with a tomato and cucumber salad with fresh basil and Italian vinaigrette. For dessert serve almond biscotti.