Peel the kohlrabi. This recipe uses the white turnip-like flesh of the vegetable. Cut the flesh into long matchsticks about 5 mm (¼ inch) thick. Transfer the kohlrabi matchsticks to an airtight container and sprinkle with a little salt. Cover and set aside in a cool place overnight.
This emulsion is similar to a mayonnaise. Put the wasabi, egg yolk, lime juice, honey and salt into the bowl of a food processor. Turn the machine onto a low setting and slowly, very slowly, pour 150 ml (5 floz) of oil into the food processor in a steady stream. At this stage, if the mayonnaise gets a little thick, add a little splash of water. Slowly pour in the rest of the oil in a steady stream. The end result should resemble a really thick mayonnaise.
Add all ingredients in a bowl with 130 ml (4¼ fl oz) water and mix until well combined. Set aside.
When you are ready to serve the sashimi, use a sharp knife to cut each piece of tuna into 7– 8 pieces and transfer to a bowl. Whisk the vinaigrette and pour about 3 tablespoons of it over the tuna. Leave to marinate for 1–2 minutes.
In another bowl, add the spring onions, pickled kohlrabi (drained), salted cucumber and wakame. Gently mix the lot with your hands and divide among five bowls.