A jar worth keeping in the fridge

There is a particular pleasure in a condiment that costs pennies, takes ten minutes and improves almost everything it touches. Pickled red onions are that condiment. One large onion, a good slosh of vinegar and a spoonful each of sugar and salt will give you a jar that sits happily at the back of the fridge for weeks, ready to lift a plain cheese sandwich, a bowl of grain salad or a plate of cold roast lamb.

The transformation is partly chemistry and partly theatre. Vinegar softens the onion's blunt, eye-watering bite into something cleaner and sharper, while the anthocyanins in the red skin bleed into the liquid and turn the whole jar a startling, luminous pink. Leave them for half an hour and they are perfectly pleasant. Leave them overnight and they become something else entirely: tender, glassy, sweet-sour and quietly addictive.

What you actually need

The proportions matter more than the ingredients, and the ingredients are deliberately modest.

• Red onions — one large onion fills a 500ml jar once sliced; two if you are feeding a crowd.
• Vinegar — 150ml. White wine vinegar is the best all-rounder. Cider vinegar gives a softer, fruitier note, distilled white is sharp and cheap, and malt vinegar brings a robust, chip-shop earthiness that suits cold meats. Rice vinegar is gentler still if you want something mild.
• Sugar — 1 tablespoon of caster sugar. Honey or light brown sugar work too, adding a faint caramel edge.
• Salt — 1 teaspoon of fine sea salt, which seasons the onion from within rather than just on the surface.
• Water — 150ml, optional but recommended. All-vinegar pickles are bracing; cutting the vinegar with water gives a rounded condiment you can eat by the spoonful.
• A clean jar — a 500ml Kilner or a washed jam jar with a tight lid. Glass is best, as the pink liquid will stain plastic.

Two ways to make them

The cold method is the workhorse. Slice the onions as thinly as you can manage — 2mm is the target, and a mandoline earns its keep here — then pack them loosely into the jar. Add the sugar and salt, pour over the vinegar and water, and give the jar a firm shake. Leave it on the worktop for 30 minutes, then refrigerate. The onions will be ready to eat within the hour but are at their best after 24.

The hot method gives a softer, more deeply flavoured pickle. Warm the vinegar, water, sugar and salt in a small pan until the grains dissolve, then pour the liquid over the sliced onions while it is still steaming. Press the onions down so they are fully submerged, leave to cool, then lid and chill. They are ready in about an hour and the texture stays pleasingly yielding rather than crunchy. Do not boil them hard — a brief simmer is plenty, and anything longer turns them to mush.

Flavours to ring the changes

A plain pickle is a fine thing, but the jar is also an excellent place to use up odds and ends from the spice shelf.

• Bay leaf and black peppercorns — the classic pairing, especially good alongside roast beef.
• Star anise and a strip of orange peel — a winter version that sings with cold ham or a Boxing Day sandwich.
• Mustard seeds and a dried chilli — adds a gentle warmth that cuts through rich cheese.
• A sprig of thyme or rosemary — lovely with lamb, and a good use for the woody stems left at the end of the bunch.
• Shallots instead of onion — smaller, sweeter and slightly milder; pickle them whole if they are tiny.

Where they earn their keep

Sandwiches are the obvious home. Mature Cheddar, thick-cut ham, leftover roast chicken, smoked mackerel, a hot fish finger roll, halloumi straight from the pan — all of them improve with a forkful of pink onion tucked inside. Beyond that, they are the sort of thing that quietly upgrades a whole plate: scattered over a tomato salad in late summer, stirred through a warm potato salad, piled onto a jacket potato with soured cream, or laid over a burger that needs cutting through.

They also do fine work at the edges of a meal. A ploughman's lunch without them feels unfinished, and they make a bowl of leftover grains taste deliberate rather than dutiful. If you grow your own, the same liquid will pickle thin slices of beetroot, radishes, fennel or cucumber, so the jar rarely sits empty for long.

Keeping them, and a few small snags

Stored in the fridge with the lid tight, pickled red onions keep for three to four weeks and often longer. Use a clean fork each time rather than tipping the jar, and keep the onions pressed beneath the surface of the liquid — a scrunched-up square of baking parchment tucked on top does the job beautifully.

If the liquid starts to fizz, or the onions turn slippery or smell yeasty rather than sharp, bin the lot. Colour will fade slightly over time but the flavour deepens, and the leftover vinegar is far too good to waste: whisk it into a salad dressing, splash it into a pan of braised cabbage, or use it to deglaze the roasting tin after a joint of pork.