Caramelising onions

Why it takes so long

Onions are around ninety per cent water. Until that water has gone, the pan cannot rise above 100°C, and the browning reactions that produce the colour and the deep sweet flavour do not run below about 140°C.

So the first twenty minutes or so are entirely evaporation. Nothing much appears to happen: the onions go translucent, then soft, then start to reduce in volume. Only once they are genuinely dry does the browning begin, and then it takes another twenty to thirty minutes of patient, low-heat cooking for the sugars and amino acids to build the dark, complex flavour that is the point.

Turning the heat up does not compress the timeline; it scorches the edges while the middle is still wet, and scorched onion is bitter. There is no version of this that takes ten minutes.

The method

1. Slice a large quantity of onions — they will reduce to about a fifth of their starting volume, so use twice as many as you think.
2. Use the widest, heaviest pan you own. Width means evaporation area.
3. Medium-low heat, a generous amount of fat — butter, oil or both.
4. Add salt early. It draws water out and speeds up the first phase noticeably.
5. Cook, stirring every few minutes. For the first twenty minutes they will simply soften and shrink.
6. When the base of the pan starts browning, add a splash of water and scrape it back into the onions. Repeat this whenever it browns — it is the single most useful trick and it both prevents burning and puts the flavour back.
7. Keep going until they are deep brown and jammy. Forty-five minutes to an hour from the start.
8. Finish with a splash of vinegar or balsamic, which lifts the sweetness and stops them tasting flat.

The shortcuts, and their cost

Bicarbonate of soda

A small pinch raises the pH and browning runs faster in alkaline conditions. It genuinely speeds things up — and too much makes the onions mushy and can leave a soapy taste. An eighth of a teaspoon per large onion at most.

Sugar

Adds sweetness and colour quickly. It is not the same flavour; you get sweet onions rather than deeply savoury ones.

A lid for the first phase

Traps steam and speeds up the softening, then must come off for the browning. Genuinely helps and costs nothing.

A pressure cooker

Softens them fast, then finish uncovered. Works, and needs two pans.

Higher heat

Does not work. Scorched edges, wet middles, bitter result.

A big batch, frozen

The best answer. Make an hour's worth once and freeze it in portions.

What to do with them

• French onion soup, which is essentially caramelised onions and stock.
• On a burger, a sandwich or a hot dog.
• Folded into mashed potato, a risotto or a frittata.
• On a tart or a pizza with goat's cheese or anchovy.
• Stirred into gravy or a pan sauce, where they add sweetness and body.
• As a base for a curry or a tagine, where the depth is doing real work.

Common questions