Cook spaghetti according to package directions; drain. Meanwhile, cut off and discard root end of bok choy. Cut leaves from stalk; coarsley chop and set aside. Cut stalks into 1 inch pieces. In a large skillet or wok, stir-fry chicken in 1 tablespoon canola oil until no longer pink. Remove and keep warm. Stir-fry the bok choy stalks, celery, peppers and onion in remaining oil for 4 minutes. Add bok choy leaves; stir-fry for 2-4 minutes longer or until vegetables are crisp-tender. Stir in teriyaki sauce. Add chicken and spaghetti; heat through.