Herring in Oil with Onion (Śledź w oleju z cebulą)

// SUPPLY LIST

// OPERATIONS

1. 1 Rinse the herring fillets under cold running water, rubbing off any surface salt crystals and scraping out the black membrane lining the belly flap with a thumbnail — that membrane is the single biggest source of bitterness in a home jar, and no amount of soaking removes it later. Run a fingertip from the head end toward the tail to find stray pin bones and pull them with tweezers in the direction they lie; if the fillets still carry skin, grip the tail end and peel toward the head while the fish is fridge-cold, and it lifts away in one strip.
2. 2 Taste a scraping of the raw fillet first and let it set the clock, because the two things sold as salted herring are not the same fish: hard barrel-salted fillets are fiercely salty and want 10-12 hours in water, while matjes-style (matiasy) are already mild and want only 2-3 hours. Lay the fillets flat in a glass or ceramic bowl — never aluminium or bare steel, which salt and acid pit, giving the fish a metallic edge — cover with about 1 litre of cold water, refrigerate at 2-4°C / 36-39°F, and pour off and replace the water three times across the soak, spacing the changes evenly (at roughly 3, 6 and 9 hours for a 12-hour soak, or hourly for a short one). Leaving them in one unchanged bowl is the classic failure: the water saturates within a couple of hours and the fish then just sits in its own brine, coming out as salty as it went in.
3. 3 Drain, cover the fillets with cold whole milk — about 750 ml, enough to stand 1 cm over them in a snug bowl — and refrigerate a further 6-8 hours for barrel-salted fillets or 2-3 hours for matjes; milk does what water cannot, drawing out the last of the salt while keeping the flesh soft and faintly sweet rather than flabby and blank. Test before going on: cut a 5 mm sliver from the thick shoulder end and eat it, and it should taste seasoned like a good cured anchovy, savoury and clean, with no sting at the back of the throat. If it still stings, give it two more hours in fresh milk, but do not push total soaking past 24 hours, because over-desalted herring turns cottony and no amount of oil brings it back.
4. 4 Whisk the 15 ml of 10% spirit vinegar into 250 ml cold water, submerge the drained fillets for exactly 15 minutes, then lift them out and pat them bone dry on both sides with paper towel. The short bath firms the surface, rinses off milk residue and lifts the flavour, but do not use neat vinegar or extend it to an hour: the acid then denatures the protein all the way through and you get opaque, chalky white flesh instead of the translucent grey-pink silk that defines this dish. Drying matters as much as the bath — water carried into the jar pools under the oil, clouds it and halves the keeping time.
5. 5 Lay each fillet on a board with the side the skin came off facing down and cut across the width at a slight angle into 2-2.5 cm pieces, discarding the last 3 cm of thin tail, which is mostly skin and fine bone. Cutting on the angle rather than straight down gives a broader face that holds the oil and spices, and pieces much larger than this never season through to the middle in three days.
6. 6 Slice the onions 2 mm thick with a sharp knife rather than a mandoline, which crushes the cell walls so they weep and go slimy under the oil. Raw is the traditional treatment and the one you want if the onion is mild — bite a sliver to check. If it is aggressive, the accepted coastal home fix is to sit the slices in a sieve and pour a kettle of boiling water (100°C / 212°F) over them for no more than three seconds, refresh at once under cold water and dry thoroughly; any longer in the heat and they collapse into something sweet and soggy, and any water left clinging will cloud the oil.
7. 7 Build the jar in layers in a 1.5 litre glass jar, or two 750 ml jars, or a glass or ceramic dish: a thin bed of onion, then a single non-overlapping layer of herring, then a scatter of bay, allspice, peppercorns, mustard seed if using and a few grains of the sugar, repeating until everything is in and finishing with a layer of onion. Do not try to force this into a 1 litre jar — 500 g of fish and 250 g of onion fill the best part of a litre before any oil goes in, and a jar packed to the shoulder cannot be topped up. Distributing the spices through the layers rather than tipping them on top is what makes every piece taste the same on day three, and the half teaspoon of sugar is not there for sweetness but to round off the vinegar's edge.
8. 8 Pour the 400 ml of room-temperature rapeseed oil slowly down the inside of the jar until everything is submerged by at least 5 mm, then tap the jar firmly on the worktop and run a skewer down the sides to release trapped air pockets — the oil has to fill the gaps between the layers before it can stand above them, so expect the level to drop as you work. Any piece of herring or onion left sitting proud of the oil line will oxidise, grey off and taste rancid within days. Use rapeseed or sunflower oil only — olive oil is wrong here, its own flavour fights the fish and it goes cloudy and waxy at fridge temperature, setting to a semi-solid paste around the pieces.
9. 9 Seal and refrigerate at 2-4°C / 36-39°F for 48-72 hours, turning the jar over once a day. This wait is the recipe, not a suggestion: on day one the onion is still raw and the fish tastes like a separate ingredient, by day two the bay and allspice have moved into the oil, and on day three it peaks, the flesh silky right through and the oil faintly amber and perfumed. Fully submerged it keeps up to 10 days in the fridge and you should top up with fresh oil whenever you take pieces out — but keep it genuinely cold and never leave the jar standing at room temperature, because this is a lightly acidified preserve and refrigeration is doing the safety work.
10. 10 Shortly before serving, start the potatoes in a large pan of cold, generously salted water, bring to a bare simmer at 90-95°C / 194-203°F and cook 18-22 minutes from the point it starts to tremble, until a knife slides into the centre with no resistance but the outer edges have not begun to fray. Drain, return them to the dry hot pan off the heat for two minutes to steam off — this is what makes them floury rather than waterlogged — then toss with the butter and most of the chopped dill.
11. 11 Take the herring out of the fridge 30-40 minutes before serving and let it come up to 16-18°C / 61-64°F: straight from the fridge the oil is sluggish and thick and mutes everything, while at cool room temperature it runs clear and coats each piece. Lift the herring out with a fork, letting the excess drip back into the jar, and arrange it on a cold plate with plenty of the onion, a spoonful of the spiced oil over the top and a pinch of the reserved dill.
12. 12 Serve the cold herring against the hot buttered dill potatoes, with dark rye bread on the side and the vodka straight from the freezer in small glasses — the contrast between cold oily fish and steaming floury potato is the whole point of the dish, and it is why this exact plate turns up on Wigilia tables and at every name-day party in Poland.

// CLASSIFICATIONS