In a large nonstick skillet, stir-fry the chicken, kielbasa, onion and garlic in oil until onion is tender. Add the peppers, mushrooms, tomatoes, herbs, Cajun seasoning, salt and pepper. Cook and stir until chicken juices run clear and vegetables are crisp-tender.
Combine cornstarch and cold water until smooth; add to the skillet. Bring to a boil; cook and stir for 2 minutes or until thickened. Serve over spaghetti.