Prepare the seafood: clean the oysters, shrimp, and mussels thoroughly. Remove the beards from the mussels. Rinse the shrimp in salt water.
2
Make the spicy seasoning: mix 1/4 cup gochugaru with 2 teaspoons sesame oil and about 1/4 teaspoon black pepper, stirring until the chili flakes are evenly coated with oil.
3
Prepare vegetables: mince the onion and garlic, slice the king oyster mushroom, and cut the white parts of the green onion into pieces, reserving the green tops for garnish.
4
Heat oil in a pot and sauté the onion, garlic, white parts of green onion, and mushroom slices until fragrant.
5
Add 2.5 cups of anchovy and kelp broth, cover, and bring to a boil. Simmer for 5 minutes to develop the flavors.
6
Add the oysters and shrimp to the broth. Add the mussels, discarding any that do not close properly.
7
Break the soft tofu into pieces and gently press them into the broth. Season with 2 tablespoons of fish sauce.
8
Stir in about half of the spicy seasoning mixture, reserving the rest for later.
9
Cover and simmer over medium-high heat for 5-6 minutes until the mussels open, the shrimp are cooked through, and the broth is deeply flavored.
10
Remove from heat and crack an egg into the pot, allowing it to cook partially in the residual heat so the white sets but the yolk remains slightly runny. Garnish with green onion tops.
11
Serve immediately in a hot bowl or stone pot while bubbling hot. Diners can add the remaining spicy seasoning to taste. Serve with rice and assorted Korean side dishes and kimchi.