Wipe the kombu to remove any dirt. Place kombu and water in a pot and let it sit for 30 minutes.

Trim the roots off the mizuna and cut into bite-sized pieces.

Cut the tofu into 4–6 pieces.

In a bowl, mix the oysters with potato starch. Add water and gently wash the oysters, changing the water until it becomes clear.

Add the tofu to the pot with kombu and heat over medium heat. Remove the kombu just before the water boils, then add the oysters and simmer on low heat for 3–5 minutes until cooked. Add the mizuna and cook briefly.