In a large nonstick skillet coated with cooking spray, brown chicken in 2 teaspoons oil; drain. Remove and keep warm. In the same skillet, saute the green pepper, celery and onion in remaining oil for 3 minutes. Add the mushrooms, carrot and garlic; saute 2-4 minutes longer or until vegetables are tender.
Stir in the tomato sauce, tomatoes and seasonings. Bring to a boil. Reduce heat; simmer, uncovered, for 10-12 minutes or until thickened. Meanwhile, cook spaghetti according to package directions.
Stir chicken into tomato mixture; cook for 8-12 minutes or until chicken is no longer pink. Discard bay leaf. Drain spaghetti; serve with chicken mixture. Garnish with additional carrot if desired.