Place shiitake in a small heatproof bowl; cover with boiling water and stand for 20 mins, or until softened. Discard the woody stems of the shiitake and slice the mushrooms thinly.
Combine the scallops, shao hsing rice wine, a pinch of ground white pepper, sugar and soya sauce in a small bowl; marinate for 10 mins.
Layer the sliced shiitake and scallops alternately in 6 x ½ cup heatproof bowls or ramekins.
Place a large bamboo steamer over a wok with sufficient water over medium heat until water is steaming, but not boiling.
Very lightly whisk eggs and stock with a fork in a large bowl; season with a pinch of salt and ground white pepper.
Strain egg mixture into a jug and pour over scallop mixture in dishes. Place dishes in bamboo steamer, cover, steam custards gently for 20 mins or until just set; remove from heat.
Heat combined oils in a small saucepan until just smoking. Remove from heat; add onion to pan, then add light soya sauce. Spoon oil mixture over custards before serving.