Fish Fumet

Cut the fish frames into pieces that fit your pot. Pull out any dark gills and rinse the bones under cold running water, then soak them 10-15 minutes in a bowl of cold water to leach out blood, which would otherwise cloud and grey the stock. Drain well. The bones must be lean and white — no oily fish like salmon or mackerel, which turn a fumet rank.

Melt the butter in a wide pot over low-medium heat. Add the leek, fennel and mushrooms and sweat gently, stirring, for 5-6 minutes until soft and translucent but with no colour at all. Browning here would tint and coarsen the stock — you want the sweetness of the vegetables, not caramel.

Add the drained bones to the pot, raise the heat to medium and stir for 2-3 minutes, just until the flesh clinging to the frames turns from translucent to opaque white and stiffens. Again, take no colour. This brief stiffening sets the proteins so they release flavour cleanly rather than clouding the broth.

Pour in the white wine and scrape the pot floor. Bring to a brisk simmer and let it bubble 3-4 minutes to cook off the raw alcohol and concentrate the wine — this builds the bright backbone of the fumet. Reduce by about half before adding water.

Add just enough cold water to barely cover the bones, then tuck in the bouquet garni, cracked white peppercorns and lemon slices. Bring up slowly to a bare simmer — a few lazy bubbles breaking, never a rolling boil — and skim off the grey foam that rises. Hold this whisper of a simmer for exactly 25-30 minutes, no longer; pushed past that, fish bones turn the stock bitter and glue-like.

Ladle the fumet — do not tip the pot — through a fine sieve lined with damp muslin into a clean container, leaving the settled sediment behind. Do not press the solids, which would push grit and cloudiness through. Cool quickly over an ice bath and refrigerate; use within two days or freeze. Leave it unsalted so it can reduce freely into sauces later.