Ukha

Pull the gills out of the fish heads with kitchen scissors and throw them away.

Do not skip the gills. They hold blood and grit, they turn a broth bitter and they cloud it, and in a soup with nothing to hide behind that is the whole dish lost for two minutes of work.

Rinse the heads, frames and tails under cold running water until no more blood comes off them.

Put them in a wide pan with the 2 litres of cold water, the whole onion, the carrot, the bay leaves, the peppercorns and the allspice.

Bring it up slowly over about 15 minutes, until the surface trembles and the first bubbles start to rise.

Skim the foam off constantly during those first minutes, until the surface stays clear.

Hold it at 80-85C/175-185F, where the broth shivers and steams but no bubble breaks the top.

Never let it boil. Clarity is the point of ukha, a hard boil churns fish fat and protein into the liquid, and unlike almost everything else in cooking that cannot be undone afterwards.

Cook for 25 minutes and no longer. Fish bones give what they have quickly and then start giving glue and mud instead.

Strain the broth through a fine sieve, or a sieve lined with muslin, into a clean pan.

Discard the heads, frames and boiled aromatics without pressing them, since pressing pushes exactly the fine solids you have just strained out back into the pot.

Bring the strained broth back to a bare simmer and add the potatoes.

Cook for 10-12 minutes, until a knife tip goes into a cube with no resistance.

Use waxy potatoes and not floury ones. A floury potato breaks up and clouds the broth, which is the one thing this soup cannot afford.

Season the broth properly with fine salt now, so the fillet poaches in seasoned liquid rather than being salted afterwards.

Lower the heat until the surface is barely moving and slide the fillet pieces in.

Poach for 6-8 minutes, until the flesh has turned opaque right through and the flakes part under gentle pressure from a spoon.

Resist thickening it. There is no roux, no cream and no crushed potato here, and a broth that tastes thin is a broth that wants salt or a longer skim rather than starch.

Take the pan off the heat and stir in the vodka if you are using it.

Scatter the dill and parsley over the pan.

Ladle carefully into warm bowls, keeping the fish pieces whole.

Serve with the lemon wedges, squeezed in at the table by anyone who wants them.

Eat it the day it is made. Reheating breaks the fillet into threads and clouds the broth, and a cloudy ukha has lost the only thing it was built for.

Cool any left over in a shallow container, refrigerate within two hours and eat it cold or barely warmed the next day.