Source: Mrs Beeton's Book of Household Management (1861)
Salt the thick parts of the head and shoulders — the flesh along the spine and inside the gill cavity — about 1 to 2 hours ahead. This draws moisture to the surface through osmosis, concentrating flavour and firming the protein structure so the delicate flesh holds together during poaching.
Lay the fish in a fish kettle or large shallow pan and cover with cold water. The cold start matters: it allows the external flesh to cook gently before the internal temperature rises, preventing the outside from toughening before the centre is done. Pour the water in at the side of the kettle, not over the fish itself — turbulence will shatter the already-fragile head and shoulder meat. Add salt at the ratio of 5 ounces per gallon (140 grams per 4.5 litres), which approximates seawater salinity and seasons the flesh evenly from the outside in.
Bring the water to a simmer gradually — this takes 15 to 20 minutes depending on the mass and your heat source. Once small bubbles break the surface consistently, skim thoroughly. You'll see a grey-white scum rise; this is denatured protein and blood, and clearing it prevents a bitter, muddy flavour in both the fish and the cooking liquor. Skim twice more over the first 5 minutes as fresh scum surfaces. Lower the heat so the water barely trembles — a rolling boil toughens the exterior before the heat penetrates to the bone. Simmering at this gentle pace takes 20 to 30 minutes for a 1.5 kilogram head and shoulders, depending on thickness. The flesh should flake easily at the thickest point near the spine but not fall apart.
Lift the fish carefully onto a cloth to drain — the residual heat will finish any carryover cooking. Serve while still hot, garnished with lemon wedges, fresh horseradish, any roe or liver from the cavity (which you've kept), and a knob of cold melted butter alongside, or oyster sauce if you prefer. The cooking liquor keeps for stock.
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