If making kombu stock, soak a 3''x4'' piece of kombu in 2 1/2 cups of water overnight, or place kombu in a pot with 2 1/2 cups of cold water over medium heat. When the water comes to a near boil, remove kombu.

In a pot, heat the stock. Add shrimp and crab meat. Stir and cook for 30 seconds. Add 1 teaspoon of fish sauce and black pepper, stir and cook for about a minute.

Pour the stock mixture into a pot. Bring to a simmer. Add the remaining fish sauce (more or less to taste).

Whisk the cornstarch/tapioca starch with 1 1/2 tablespoons of water and drizzle the mixture slowly into the pot. Gently stir while adding the mixture until the soup is thickened to your liking.

Cook for 1-2 more minutes. Add black pepper and sliced scallions, stir and then transfer to serving bowls. You can add more scallions and cilantro to each bowl.