STEP 1 Combine 4 tablespoons cornstarch, 1 tablespoon each soy sauce and rice wine (or sherry) and egg white in a bowl. Add chicken and stir to coat.
STEP 2 Combine the remaining 1 tablespoon each cornstarch, soy sauce and rice wine (or sherry), water, hoisin and rice vinegar in a small bowl. Set aside.
STEP 3 Heat 2 tablespoons oil in a wok or large cast-iron skillet over high heat. Add the chicken; cook without turning, breaking up stuck-together pieces, until golden on the bottom, 2 minutes. Stir; continue cooking until golden on all sides, 1 to 2 minutes more. Transfer to a plate.
STEP 4 Add the remaining 1 tablespoon oil, scallions and garlic. Cook, stirring, until fragrant, about 15 seconds. Add peas; cook, stirring often, until bright green, 2 to 3 minutes. Add the reserved sauce mixture; cook, stirring, until thick, about 1 minute. Return the chicken to the pan; cook, stirring, until heated through, about 1 minute more.