2.Place pork on wire rack in large shallow baking dish; brush all over with ¼ cup of the char siu sauce. Roast, uncovered, in moderate oven about 40 minutes or until cooked as desired, brushing occasionally with pan drippings. Cool 10 minutes; chop pork finely.
3.Meanwhile, heat peanut oil in wok; stir-fry chicken, garlic and mushrooms for 5 minutes. Add water chestnuts, chopped onion, sauces, sesame oil, pork and remaining char siu sauce; stir-fry until chicken is cooked through. Remove from heat; add sprouts, toss sang choy bow gently to combine.
4.Divide lettuce leaves among serving plates; spoon sang choy bow into leaves, sprinkle each with sliced onion.
An easy way to separate whole lettuce leaves is to rap the head of the lettuce, stem-end down, on a hard surface to loosen the core. Discard the core, and run a strong stream of cold water into the cavity the leaves will fall away, intact. Submerge the leaves in iced water until ready to serve; use the lettuce centre for a salad.