Sweating and caramelising onions

Why

Onions are mostly water, and until it has evaporated the surface of the slices can't get much above 100 °C: over gentle heat they simply soften, turn translucent and lose their sharp bite – that is sweating. Only once the water has gone do the sugars start reacting with amino acids in the Maillard reaction and caramelising on their own, producing a golden-brown colour and a sweet, deep flavour. Properly caramelised onions take roughly half an hour to an hour over low to medium heat, depending on the amount; over high heat the edges brown and burn before the centre has softened, and burnt onion turns bitter. A pinch of salt at the start draws water out of the cells by osmosis, so the onions release their juice and soften faster.

How to do it

• To sweat onions, use gentle heat and a little fat, and stir until soft and translucent but not browned.
• Add a pinch of salt right at the start so the onions give up their water faster.
• For golden onions, allow half an hour or more, and raise the heat only once the water has evaporated.
• Dissolve the brown film on the bottom of the pan with a spoonful of water and stir it back in – it's flavour, not burning.
• Use a wide pan so the water has room to evaporate, and stir more often the darker the onions get.

If it goes wrong

Pick out a few burnt bits; if the whole batch tastes bitter, it can't be saved – neither sugar nor cream will reliably mask it, so start again over lower heat. Onions that won't colour are still wet: uncover the pan and let the water cook off.