- Soak the oysters in the water for 1-2 hrs, then drain, reserving 375ml of the soaking water. Set aside.
- Soak the black sea moss in the water for several mins to soften, then rinse in several changes of water and squeeze dry.
- Bring a saucepan of water to a boil. Add the black sea moss, 1 tsp of the oil and 2 slices of the ginger, and boil for 20-30 mins. Remove from the heat and drain.
- Heat the remaining oil in a wok over medium heat until very hot. Stir-fry the remaining ginger and spring onion for about 1min. Add the oysters and stir-fry for 1 more min. Season with the rice wine, oyster sauce, soy sauce, sugar and liquid from the soaked oysters. Bring the mixture to a boil, simmer for about 20 mins and remove from the heat. Discard the ginger and spring onion, drain the oysters and reserve the sauce.
- Line a wide-rimmed heatproof dish with the cooked oysters in a single layer and fill the center with the sea moss. Pour the reserved sauce over the oysters and cover the bowl with aluminium foil. Steam the dish in a covered wok or steamer for 1-1½ hrs. Remove from the heat and drain all the sauce into a small bowl. Place a serving platter over the dish of oysters and sea moss, and carefully invert them onto the serving platter. Arrange the blanched lettuce leaves around the dish.
- Bring the reserved sauce to a boil over high heat in a saucepan. Add the salt, pepper, sesame oil and cornstarch solution, and mix until the sauce thickens. Remove from the heat, pour the sauce over the dish and serve immediately.