Bring the water to a boil. Add the galangal, lemongrass, kaffir lime leaves, and shallots, then add 1/2 teaspoon of salt.

Once the water is boiling rapidly, add the sea bass pieces. Use high heat. Do not cover the pot or stir, as this will keep the fish from becoming fishy and will help it taste better.

When the fish is cooked, reduce the heat to medium. Add the king oyster mushrooms, then add the green onions and cilantro. Once the mushrooms are cooked, remove from heat.

Lightly crush the Thai chilies and put them in a bowl. Squeeze in the lime juice, add fish sauce, and balance the flavor with a little sugar. Taste to make sure it's mainly sour and spicy.