Wipe the kombu with a dry cloth to remove any dirt, then cut into 3cm-wide pieces. Soak the kombu in 450ml water in a pot for about 1 hour.

Cut the firm tofu into 1cm cubes. Soak the dried wakame in water to rehydrate, then drain and cut into bite-sized pieces if necessary.

Heat the pot with kombu over low heat to slowly extract its umami. Remove the kombu just before the water boils.

Bring the water to a boil, then turn off the heat and add the bonito flakes. Let them steep for 5-6 minutes until they sink to the bottom.

Strain the broth through a sieve lined with kitchen paper into a bowl to remove the bonito flakes.

Return 400ml of the strained broth to the pot. Add the rehydrated wakame and bring to a boil. Then add the diced tofu.