Soak the clams in salted water for at least 20 minutes. Meanwhile, put the sesame oil and vegetable oil in a medium, heavy-based saucepan (ideally a cast-iron one) on a low-medium heat.
Add the spring onion and onion, and fry just until they start to sizzle.
Add the gochugaru and minced garlic, and stir well for about a minute, taking care that they do not catch and burn.
Pour in 450ml water and turn up the heat to medium-high. When the soup starts to simmer, add the soy sauce, fish sauce, sugar, drained clams and king prawns.
After a couple of minutes, when the clams start to open, gently lay the block of silken tofu in the broth and cook for another two to three minutes.
Take the pan off the heat, season the broth with fine salt, if need be, then turn off the heat. Season with fine salt, if need be.
Carefully crack in the egg, then finish with a drizzle of sesame oil, some more sliced spring onion and black pepper to taste.
Serve in the pot with a bowl of steamed rice on the side.
