Cut the king oyster mushrooms and rice cakes into bite-sized pieces.
Slice the tofu and green onions as well.
Add kelp and water to a pot to make broth.
Remove the kelp, then add rice cakes, king oyster mushrooms, and tofu to the pot and bring to a boil.
Season with soy sauce and salt to taste.
Add green onions, let it boil briefly, then serve in a bowl.