As promised, here’s more on what I think is the most intriguing and flavoursome food ingredient I’ve ever come across: katsuobushi.

Along with dried konbu seaweed, katsuobushi is the essential ingredient in the Japanese ‘stock’, Ichiban, or ‘number one' dashi which, in turn, is the cornerstone of traditional – and much modern – Japanese cuisine. It is packed with amino acids to give a splendid umami-flavour punch, and is slowly gaining recognition among top chefs in the West, like Ferran Adrià and Heston Blumenthal (who has become something of an expert on umami). 

But what exactly is it?

(This is my precious supply. So precious is it, in fact, that it sits firmly in my ‘Too good to use’ drawer alongside various other ingredients I have bought when travelling and am saving for a ‘special occasion’ which, deep in my heart, I know will never come, so that really, these things exist merely to be taken out of the drawer once in a while to be shown to admiring friends, before being put away again until, eventually, they get too old to use yet still I don’t throw them away because, you know, just in case… Hence it's still in its plastic packaging, like Ruby Wax's parents' sofas).

Though they look, feel and – when you bang two together – sound exactly like varnished wood (like, say, something you might use to make holes in soil for planting, or a midget’s chair leg), these are in fact fish fillets: from a bonito fish, to be precise.

An oily, fast moving, silvery-black fish, related to the tuna, the bonito is caught in seas around Japan and as far south as the Philippines. Sometimes the Japanese eat bonito as sashimi, or lightly grill it, but generally they think of it as inferior to other sashimi-friendly fish because it has a slightly stronger, funkier flavour. And besides, they have a much smarter use for it.

When I was researching my book, Sushi and Beyond (out late May), I visited one of the most renowned bonito processing plants, in Yaizu, south of Tokyo, where I saw teams of local ladies deftly filleting steamed bonito. 

The fillets are then smoke-dried (or, less often, dried in the traditional way in the sun as they've been doing since the Middle Ages – needless to say, this produces the best quality fillets), and covered with a bacteria to ferment them slightly. Meanwhile, the bones are ground to make vitamin supplements and even the boiling water is reused to make stock.

(Bonito fillets during smoking)

The result is a smooth, rock hard fillet which, to use, you need to shave on an up-turned plane called a katsuobushi kezuri.

This is mine. Pristine and unsullied by my precious katsuobushi hoard.

(Lower pic is the kezuri ready for use – the shavings fall into the box)

The best restaurants in Japan will shave their katsuobushi fillets fresh every day for maximum flavour (once you shave them, they start losing flavour immediately). For home use the Japanese buy katsuobushi flakes pre-shaved in vacuum packed bags from the supermarket – although, at Tsukiji market, there are one or two stalls that sell it freshly shaved in brown paper bags. 

Most, however, buy dashi powder granules, much as we in the West buy stock cubes.

Some of these powdered dashi products (this one is actually for finished miso soup) aren’t bad, but as with stock cubes, they will never be as good as dashi made from primary ingredients. And, of course, the powders are no good when you want to use the flakes as a garnish – they are fantastic on tofu and literally dance, hypnotically, in the heat from the classic Okinawan fast foods, okonomiyaki and tako yaki.

(Somewhere underneath those flakes are some steaming octopus balls: Mmmm).

Sadly, my own fillets are, as I said, too good to use, so I guess I will have to survive on the memory of my dinner at the private dining club Mibu, in Ginza (a favourite Tokyo haunt of Adria and Blumenthal), where the chef, Ishida-san, shaves his superb quality, sun-dried katsuobushi flakes moments before adding them to his dashi. It resulted in a soup that literally made every hair on my body stand on end. Incredible.

(The yellow stuff is chrysanthemum leaves, the white bit, hamo, or pike conger eel).

Next up: How to make the best dashi in the world – secret tips from one of the greatest chefs in Japan.