Season chicken with half the soy sauce and cayenne; let stand. In small bowl, blend remaining soy sauce, vinegar, ginger, garlic and cornstarch; set aside. In wok or large skillet, heat half the oil over high heat; stir-fry chicken until no longer pink inside, about 5 minutes. Remove chicken to a platter. Heat remaining oil over high heat; stir-fry mushrooms 1 minute. Add broccoli, bell pepper, string beans and onion; stir-fry 1 minute. Cover and reduce heat to medium; cook until vegetables are crisp-tender, about 2 minutes. Uncover and increase heat to high; add corn and the chicken. Add broth. Pour cornstarch mixture into pan; cook, stirring, until liquid is thickened, about 1 minute. Taste and correct seasoning, adding salt if desired.
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