Mix together the pork, prawns, spring onions, garlic, ginger, oyster sauce, oil and water chestnuts.
Peel off a wonton wrapper and, with a pastry brush, lightly brush round the outside with cold water. Place 1 tsp of mixture in the centre and scrunch up into a pouch, squeezing to seal. Do not overfill. Repeat until you have around 35-40. (These can be chilled for up to 1 hr on a tray lined with baking parchment.)
For the sauce, mix together the soy sauce, sherry and ginger in a small bowl. To steam, bring a large pan of water to the boil. Line a bamboo or metal steaming basket with lettuce leaves to fit.
Arrange half the pouches in the basket (without them touching), cover and steam for 5 mins. Remove with tongs to a platter and steam the rest – check the water level. Serve, sprinkled with spring onion or sesame seeds, with the bowl of sauce.