Halibut in Japanese fish broth

Kombu (dried kelp) katsuobushi (shaved dried bonito) dashi (stock). Frozen vegetables heated briefly in the broth. Halibut poached briefly in the broth.

The technique here uses two pots to produce ichiban dashi (first stock). Usually the technique described below will be followed twice with an addition of water for niban dashi (second stock) for a clear stock base with a broad range of uses. The result is extraordinary and reminiscent of the sea.

The white dust on the kombu is mineral bloom. It is entirely natural. The excess white dust is wiped off the kombu with a clean damp kitchen cloth. This particular kombu is not heavily coated so that step is omitted. The kombu is soaked in cold water and brought to a boil. The pot is removed immediately from the heat source so the kombu never actually boils. The kombu rehydrates rapidly. In this case, the kombu was allowed to soak in the cooling water for 10 minutes. So the resulting liquid amounts to a kombu tea. The kombu is removed.

A small package of katsuobushi is added to the liquid which is brought back to the boil. Any foam that forms is skimmed off. The heat cut off again within 30 seconds and the liquid allowed to cool again. The katsuobushi sinks to the bottom of the pot.

Here's where the second pot comes in.  The used katsuobusi flakes are strained out, the liquid into the second pot. Now you have a liquid that is flavored with kombu and with katsuobushi, so one, a sea-vegetable, and two, the essence of  smoked and dried and shaved skipjack tuna.

You also have a floppy wet piece of kombu and a pile of used strained katsuobushi that can be used for a second round of dashi. If you choose to produce a second broth then add half the original amount of water, return the kombu, and return the used katsuobushi flakes with an additional amount of fresh katsuobushi flakes amounting to half the original amount used. Bring to a boil again, cool again, strain again, and  -- Tadaaaa! -- niban dashi.

But we're not doing that because we have a big fat piece of flavorful halibut that will do to the liquid what the second round would do.

The vegetables are reheated in the broth which is brought back to the boil a third time. The vegetables are strained from the liquid, which pours back into the first pot. The vegetables are tipped  into a serving bowl and covered.

The ichiban dashi now flavored with vegetable is brought to a near boil a fourth time. The halibut is placed in the hot fish and vegetable stock and allowed to cook off the heat while the liquid cools down. In this case, the final reheating of the stock was up to 170℉/77℃ so well below boiling, and cooked the fish fillet as it cooled to finish within five minutes to 140℉/60℃, which is a very nice serving temperature.

The gently cooked fish fillet and its broth, now flavored with kombu and katsuobushi, vegetables, and halibut, is placed in the bowl awaiting with the vegetables.

Now, this seems a lot of back and forth with heating this liquid, and it is, but the steps described above can be combined. The katsuobushi doesn't have to be strained out, it's perfectly edible if you do not mind little bits floating around. I've done that before. The kombu and katsuobushi can be heated together for a less than perfect result, in fact, it can all be done at once and you will most likely be satisfied with the result, but this careful technique described here provides control over each element.

Kombu and katsuobushi in convenient little packages ↑. 

A small piece of kombu with very little mineral bloom ↑. You will see this packaged in large sheets, usually regular A4 sheet paper size, but it also comes in much larger sheets. This is the stuff you will see Iron Chef Morimoto use in rehydrated form to wrap fish fillets. It is a source of glutamic acid, the origin of MSG, isolated by Kikunae Ikeda and patented by Ajinomoto Corporation, and  it is the origin of the term for the flavor sensation 'umami'.  All of this should tell you that no additional salt is necessary. 

A very small pile of katsuobushi ↓, dried and usually fermented, then shaved skipjack tuna. You will see chefs use this by the handful. An excellent product, it has several uses beyond fish stock. It can be purchased in large bags. Otherwise, the dried tuna can be purchased separately and shaved at home. The shavers are boxes with a mandolin-type shaver on top and pull drawer in front. The shavings fall into the box and are collected in the drawer. Search in images [bonito shaver] to see what I am talking about. 

ARTS !