Soak black moss overnight in a big bowl of water. Repeated rinsing with water for few times to remove impurities. Drain and squeeze dry.
Mix black moss with seasoning. Steam for 30 minutes and set aside.
Soak fish maw overnight in water. Boil a pot of water with salt, ginger and scallion, add fish maw and bring to boil with lid on high heat for 10 minutes. Off fire and let it sit for at least 1 hour. Take out the fish maw and set aside.
Boil Shanghai pakchoi with some salt. Arrange on plate with black moss on top.
Leave cornstarch water aside and bring soup base to boil. Add fish maw and abalone, cook over low heat for 30 minutes. Thicken with cornstarch water. Pour onto black moss and serve.