Start heating a pot of water with a few pinches of salt. Trim the bottom of 1 head bok choy (pak choy) and cut a cross on the bottom about 1cm / ½ inch deep. Prepare a bowl of ice-cold water and place it by the stove.
Reduce the heat so the water is just below boiling and place the bok choi's stems in the water for 30 seconds. Leave the leaves exposed, leaning over the edge of the pot.
Submerge the leaves and cook for 15 seconds before transferring the bok choy to an ice-water bath.
After a few minutes, remove the bok choy from the water and gently squeeze out the water. Place it in a sieve or colander over a bowl and leave to drain any leftover water.
Wash 100 g enoki mushrooms, cut off the tough base, and separate them.
Separate the leaves of the bok choy by pulling from the cut at the bottom of the stems.
Cut them roughly in half to divide the thick stems and tender leaves.
Pour 4 tbsp dashi stock (liquid, not powder) into a saucepan and add ½ tbsp mirin, 1 tsp Japanese soy sauce (koikuchi shoyu),and ¼ tsp salt. Boil for 1 minutes on medium heat, then reduce to a simmer, add the enoki mushrooms and cook for a further 2 minutes.
Turn off the heat, add the bok choy to the pan and mix.
Pour the contents of the pan into a heatproof mixing bowl and leave to cool for 2-3 minutes. Mixing gently will help it cool faster.
Take 2 pickled plums (umeboshi) and break them open to remove the pits. Use a knife to mash and chop them until they become a paste.
Add the pasted umeboshi to the bowl along with 1 tbsp bonito flakes (katsuobushi), 1 tsp wasabi paste and 1 tsp toasted sesame oil. Mix well.
Serve immediately (warm) or cool further and chill in the fridge. Upon serving, sprinkle with crushed roasted seaweed for sushi (nori) and toasted white sesame seeds. Enjoy!
Notes
Spinach and Napa cabbage are good substitutes for bok choy. Enoki can be replaced with your favorite mushroom thinly sliced.
Add wasabi paste at the very end of preparation to preserve its volatile compounds and distinctive flavor. You can also add more to taste.
If you're using especially salty umeboshi (like homemade pickled plums), skip the salt altogether.
Store any leftovers for no more than 24 hours, as the wasabi flavor diminishes quickly and vegetables continue to soften.
Pat vegetables completely dry before dressing to prevent a watery final dish.