The restaurant version marinates black cod for days. This one needs twenty minutes: spread the miso glaze on ordinary cod and finish it under a hot grill, where it caramelises in the time it takes the pak choi to cook.

Cod is one of the leanest proteins there is, so this plate carries 34g of protein for about 430 calories, rice included. The miso does the work that butter or batter usually does, and adds far more flavour.
The reason to eat more white fish that almost nobody mentions is iodine. The thyroid needs it to make its hormones, and fish and dairy are the main sources in the UK diet. A study of about 1,000 pregnant women in Bristol, published in The Lancet in 2013, found that two-thirds had iodine levels below the threshold for adequacy, and their children were more likely to score low on verbal IQ and reading at eight and nine. White fish such as cod is one of the richest sources there is. It matters for anyone cutting down on dairy: many plant milks are not fortified with iodine, so check the label. More on the thyroid in is it your thyroid or your hormones?
Miso is fermented soya, and it is salty. A serving here carries a moderate amount, which is why nothing else in the recipe is salted. Much of the miso on supermarket shelves has been pasteurised, which kills the live cultures, so treat it as flavour rather than as a source of live bacteria.
Pak choi is a brassica, a cousin of broccoli and kale, and brings vitamin C, folate and calcium. Brown rice adds fibre that white rice lacks.
Men who want more omega-3 should know that cod is lean and carries far less of it than oily fish; it complements the salmon and mackerel recipes rather than replacing them. For another quick Japanese-style meal, see the miso, tofu and edamame soup.