1. For the asparagus, snap off the woody base of the stems by bending the spears with your hands. Peel the stalks only if thick and tough. Put the spears into a large and shallow saucepan, cover with boiling water, add a little salt and bring back to the boil. Cook the spears for 2 minutes, drain and set aside, keeping warm.
2. For the sauce, grind the sesame seeds in a pestle and mortar, scrape into a bowl and whisk together all the other ingredients. Chop the onion and celery into a very fine dice and add to the sauce bowl. Drain the tuna and put into the sauce mix the sauce, breaking up the tuna with a fork.
3. Cut the asparagus spears in half and mix with the sauce. Serve as a starter or a side dish in a Japanese meal.