Make dashi broth by boiling water, soy sauce, tuna sauce, tablet seafood stock, mirin, rice wine vinegar. Cook until the tablet is fully dissolved. Turn off the fire, and add gamtae to the broth. Whisk to incorporate gamtae into the dashi. Let cool.
Soak the dried mushroom in warm water for about 20 minutes, or until plumped. Squeeze out water.
Crack 3 eggs, and whisk in the dashi broth.
Divide the egg mixture into 4, in a heat-proof dish such as ramekin, and add the mushroom in each portion. Cover individual dish tightly with a plastic wrap.
Cook the egg custard in a steamer, for about 15 minutes, or until the custard is set and jiggling.
Garnish the custard with small amount of scallion, and serve as quickly as possible.