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There’s a reason onions turn up in the first step of so many recipes, from French onion soup to a twenty-minute dal. They’re cheap, they keep for weeks, and they build a savoury base that’s hard to fake. A kilo of yellow onions costs less than a takeaway coffee in most cities, and it will flavour a dozen meals.
The onion aisle is more varied than it looks, though. Pick the wrong one for a salad and you get a harsh, sulfury bite that drowns everything else. Pick the right one for a long braise and it almost melts into the sauce. Here’s what each type does best, what you actually get nutritionally, and how to cook them so they taste sweet instead of sharp.
The onion family, sorted by what they’re good at
Yellow onions: the workhorse
Brown-skinned, dense, with a sharpness that softens the moment heat hits them. If a recipe just says “onion”, this is the one. They hold up through a forty-minute caramelisation and they’re the standard base for stock, stews and tomato sauce. Buy them in bulk.
Red onions: for crunch and colour
Milder raw than yellow but sharper than the sweet varieties. Their pigments are water-soluble, so a five-minute soak in cold water takes the edge off without turning them an unappetising grey. Best raw: thin slices in salads, on burgers, or quick-pickled in vinegar with a pinch of sugar and salt, ready in about twenty minutes.
White onions: the salsa staple
Sharper and cleaner-tasting than yellow, with a thinner skin and a shorter shelf life. Dice them fine for salsa, guacamole and kebabs, or pickle them whole. Use them within a month.
Sweet onions: Vidalia, Walla Walla, Texas 1015
Higher water and sugar content, lower sulfur compounds, which is why they’re mild enough to eat in thick raw slices. They bruise easily and don’t keep long, two to four weeks rather than months. Grill them, roast them in wedges, or slice them thick for onion rings. Don’t reach for them when caramelising: the extra water means you’ll steam them for twenty minutes before any browning starts.
Shallots: small, pricey, worth it
Two or three lobes to a bulb, mild and faintly garlicky. They mince into finer, less watery pieces than a yellow onion, which is why vinaigrettes and pan sauces call for them. Also the best onion for roasting whole in their skins.
Spring onions and scallions
Sold in bunches with the green tops attached. The white parts take heat well; the greens should go in at the end or stay raw as a garnish. Stand them in a glass of water in the fridge and they’ll last a week.
What you get nutritionally
One medium onion, about 110g, delivers roughly 44 calories, 10g of carbohydrate including 2g of fibre, and around 8mg of vitamin C. Modest numbers. Onions aren’t eaten for calories, though. They’re eaten for the compounds inside them, and for the fact that they make everything else taste better.
- Quercetin: a flavonoid concentrated in the outer layers, highest in red and yellow onions. Levels survive cooking reasonably well, so a long-cooked sauce still delivers some.
- Organosulfur compounds: formed when the cells are damaged. This is the chemistry behind both the tears and most of the research interest. Let a chopped onion sit for ten minutes before it hits the pan and more of these develop.
- Inulin and fructans: prebiotic fibres that feed gut bacteria. They’re also why onions cause bloating in people with IBS. Spring onion greens and garlic-infused oil are the usual workarounds.
- Vitamin C and folate: present, but water-soluble, so long boiling leaches plenty out. Raw or briefly cooked onions retain far more.
No single vegetable is a superfood, and onions aren’t either. They are a cheap, reliable way to add fibre and a lot of flavour to meals you already eat.
Cooking onions so they taste sweet, not sharp
Sweating versus caramelising
Sweating is the base step for curries, risotto and pan sauces: medium heat, a lid on, five to eight minutes, a knob of butter or a spoon of oil, and a pinch of salt. The onions go translucent and lose their raw edge without taking on colour.
Caramelising is the same onions with patience attached. Lower the heat, stir occasionally, and give it thirty to forty-five minutes. Salt early, because it draws out water. Deglaze with a splash of water whenever the pan looks dry and the fond is threatening to burn. You don’t need to add sugar; onions have plenty. A teaspoon of vinegar at the end lifts the sweetness and stops it tasting flat.
Roasting
Halve or quarter them, toss with oil, salt and pepper, and lay them cut side down on a hot tray at 220°C for 25 to 35 minutes. The cut faces go deeply brown and the interiors turn jammy. Whole unpeeled onions roasted for an hour, then squeezed out of their skins, make a silky purée for soup with almost no effort.
Raw
Slice thin rather than thick, and a mandoline beats a knife for this. Soak red onion slices in iced water for five to ten minutes to mellow them, then dry them properly or the dressing will slide off. Salt and acid them at least ten minutes before serving so they soften slightly.
Quick pickling
Equal parts vinegar and water, with a teaspoon each of salt and sugar per 250ml. Pour it hot over sliced onions, cool, and refrigerate. They’re good for two to three weeks and they’ll improve tacos, sandwiches and roast vegetables for months.
Grilling and charring
Thick wedges or whole spring onions, brushed with oil and cooked over high heat until blackened in patches. The bitterness from the char balances fatty meat better than any sauce.
Buying, storing, and cutting without tears
Choose onions that feel heavy for their size, with dry papery skins and no soft spots or green sprouts. A sprout means it has started to go, and a damp neck means rot is on the way.
Store whole onions loose in a cool, dark, ventilated spot, such as a basket in a cupboard. Not a sealed bag, and not the fridge, which is too humid and turns them soft and mouldy; cold-stored onions also go mushy when cooked. Keep them away from potatoes, which release moisture and ethylene that speeds up sprouting. Whole onions keep two to three months; cut ones go in the fridge, wrapped, for up to a week.
For the tears: use a sharp knife, since a dull one crushes more cells and releases more of the irritant. Chill the onion for fifteen minutes first. Cut the root end last, because that’s where the sulfur compounds are most concentrated. Some people swear by slicing near a running tap; a small fan pointed across the board works better.
Swapping one onion for another
Substitutions are usually more forgiving than recipe writers admit. No shallots for a vinaigrette? Mince the white part of a spring onion, or a quarter of a red onion, as finely as you can. Sweet onion missing for a salad? Soak red onion in cold water for ten minutes instead. Out of yellow onions for a stew? White works fine, and red will tint a pale sauce pink but taste perfectly good.
Spring onions in a stir-fry can be replaced with a small diced shallot or a couple of tablespoons of finely chopped leek. The one swap worth avoiding is putting sweet onions in for a caramelise, because you’ll spend an extra twenty minutes waiting for the water to cook off before any colour appears.
