THE MARINADE: Whisk the soy sauce, sherry, oil, sugar, garlic and ginger together in a medium-sized bowl until the sugar is fully absorbed. Reserve 1/2 cup of the marinade. Place the chicken in a large re-sealable plastic bag - or in two medium size bags - and pour in the remaining marinade. Press out the air and seal the bag, turning it over several times to thoroughly coat the chicken and marinate in the refrigerator for at least 2 hours - or up to 6 hours.
LEMON-OLIVE SAUCE: Combine the mayonnaise, yoghurt, lemon juice, olives and garlic, transfer to a blender or food processor and blend. Season with salt and freshly ground pepper, to taste. Transfer to a serving jug.
TOASTING THE WALNUTS: Stir-fry the walnuts quickly in a dry non-stick pan over a medium-high heat, flipping them until they are fragrant. (1-2 minutes).
GRILLING: Preheat the grill to high, cook the chicken breast halves, turning and brushing with the reserved marinade, for 15 minutes or until the juices run clear.
SERVING: Arrange the chicken on a platter, top with a little of the sauce and sprinkle the toasted walnuts over the top. Serve with the jug of Lemon-Olive Sauce, grilled tomatoes, sautéed onions and mushrooms and your favourite rice dish.