CUT the base off the lettuce, discard the outer leaves then separate the whole leaves and trim them into cups. MIX together the oyster sauce, soy sauce and sesame oil in a small bowl. HEAT a wok or large frying pan over high heat then add the oil. STIR-FRY the chicken for 3 minutes, breaking it up as you cook. ADD the mushrooms, scallions, water chestnuts, garlic, ginger and chili and stir-fry for 2 minutes. ADD the sauce mixture and stir-fry a further 2 minutes. REMOVE from heat and allow to cool a little, then spoon mixture evenly into prepared lettuce cups and garnish with extra scallion to serve.

Variations: Use ground turkey or mashed firm tofu instead of chicken. Garnish with fresh cilantro leaves and/or toasted sesame seeds. Use fresh or dried (and reconstituted) shiitake mushrooms instead of button.