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1
Set a basin of cold water outdoors. Submerge the bulging tin fully. Pierce the lid under the waterline and allow the pressurised liquid to discharge into the basin. Open the lid completely while still submerged. The water contains the spray and suppresses the initial olfactory impact. Remove the fillets and rinse them lightly under cold running water. The smell at this point is severe; this is expected and correct.
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2
Boil the potatoes in salted water until a knife meets no resistance at the centre. Drain, allow to cool slightly, peel, and slice into rounds approximately 5 mm thick. The surface should be dry and floury at the cut.
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3
Lay a sheet of tunnbröd flat. Place two or three potato slices across the centre, then lay a single surströmming fillet — or a portion of one — on top of the potato. Add a few rings of raw onion and a spoonful of sour cream. This is the assembly that decides the dish: the potato and sour cream temper the salt and acid of the fillet, the onion cuts the fat, and the tunnbröd binds the whole. Too much fish and the salt overwhelms; too little and the dish has no point. The balance is where the dish lives or dies.
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4
Roll or fold the tunnbröd tightly around the filling. The bread should hold its shape without splitting. Serve immediately, outdoors or in a well-ventilated space. The flesh, once assembled and tasted past the nose's anticipation, is salty, tangy, and umami-rich — noticeably milder than the smell predicted.