In a small bowl, use a fork to blend the chicken stock with the sherry, soy sauce and cornstarch.
Heat 1 tablespoon of the oil in a large skillet. Add the cashews and cool over moderate heat, stirring, until lightly browned, about 1 minute. Using a slotted spoon, transfer the nuts to a plate.
Heat the remaining 1 tablespoon of oil in the skillet. Add the white part of the scallions, the ginger, garlic, and Szechwan peppercorns and cook, stirringfor 1 minute. Add the sauce and arrange the chicken on a slingle layer in the skillet; season with salt and pepper. Arrange the bok choy in a layer over the chicken. Cover and cook until the chicken is cooked through and the bok choy is tender, 5 to 7 minutes. Transfer the chicken, bok choy and sauce to bowls. Garnish with scallion greens and cashews and serve with rice.