In Japanese, this type of dish is called kobujime, or “tightened with kombu”, and was originally used during the Edo period to extend the shelf life of raw fish (no refrigerators back then!). The fish was sandwiched between strips of kombu and left for a few hours up to overnight to cure. As well as lasting longer, sashimi became more flavourful after using this technique.
As it turns out, kombu is high in glutamate, one of the amino acids responsible for the 5th distinct perceived taste, umami, discovered in Japan in 1908. Anything cooked, soaked or pickled with kombu for an amount of time will have a stronger umami taste than before. So even though keeping fish cold is no longer a challenge, this technique is still used to flavour white fish and sashimi today. In the Japanese recipe below, we used the kobujime technique with asparagus, a low-calorie spring vegetable high in vitamins, minerals and fibre. And we think it’s delicious!
Use dried kombu in this recipe if you can find flat enough strips. The point is for the asparagus to be in contact with the kombu, which will be a difficult if it’s a little shrivelled or curly on the edges. If that’s the case, soak it in cold water for 5 minutes (and you can use the kombu-flavoured water as a light dashi, for example in a nimono recipe). Alternately, use kombu that has already been used to make dashi broth. The kombu is not eaten in this dish because it’s too firm, but you can reuse it by chopping it up and adding to miso soup, or drying it to use as furikake (a rice topping).