How do I cook onions without burning them?

The heat and timing for softening versus caramelising, and why fifteen minutes is not optional.

Difficulty: beginner
Time: 30 min
Read: 3 min

Short answer

Lower the heat and give it time. Softening onions takes 8 to 10 minutes on a medium heat; properly caramelised onions take 30 to 45. A pinch of salt draws moisture out and helps them cook evenly.

Burnt onions are almost always a heat and patience problem, and every recipe that says five minutes is being optimistic.

Step by step

Cut them evenly.Uneven pieces cook at different rates, so the thin ones burn while the thick ones are still raw. Consistency matters more than fineness.
Use enough fat.Enough to coat every piece. Too little and the onions in contact with the pan burn while the rest steam.
Start on medium, not high.Onions need time for their sugars to develop. High heat colours the outside before the inside softens, which reads as burnt rather than caramelised.
Add a pinch of salt early.It draws moisture out, which helps them soften evenly and stops them catching before they release their liquid.
Give softening 8 to 10 minutes.Translucent and floppy with no colour. Three minutes is not enough and it shows in the finished dish.
Give caramelising 30 to 45 minutes.There is no shortcut. Low heat, stirring occasionally, until they are deep brown and jammy. Recipes claiming ten minutes are describing something else.
Deglaze when they start to catch.A splash of water, stock or wine lifts the browned residue back into the onions. Repeat as needed — it builds flavour rather than losing it.
Use a wide heavy pan.Wide so moisture evaporates, heavy so the heat is even. A thin crowded pan burns onions in patches.
Do not crowd the pan.Too many onions steam rather than colour, then burn once the moisture goes. Cook in two batches if necessary.

Tips

Properly caramelised onions take 30 to 45 minutes. Any recipe saying ten is wrong.
A splash of water when they start to catch lifts the flavour back in rather than losing it.
Salt early. It draws out moisture and helps them soften evenly.

Common mistakes

Using high heat to speed onions up — It colours the outside before the interior softens and before the sugars develop, so you get bitterness rather than sweetness. The result tastes burnt even when it looks brown.
Believing the ten-minute caramelised onion — It is one of the most persistent recipe fictions. Genuine caramelisation is a slow process and shortcuts produce lightly browned onions, which is a different thing.

If it doesn't work

Onions browning too fast

Cause: Heat too high — Fix: Reduce the heat, add a splash of water, and scrape the base.

Onions steaming not colouring

Cause: Pan crowded or lid on — Fix: Use a wider pan, cook uncovered, or split into two batches.

Bitter taste

Cause: Genuinely burnt rather than caramelised — Fix: Nothing recovers it. Start again on lower heat.

Some pieces burnt, some raw

Cause: Uneven cutting — Fix: Cut to a consistent size.

Sticking badly to the pan

Cause: Not enough fat, or thin pan — Fix: Use more fat and a heavier pan, and deglaze as the residue builds.

Questions people ask

How long do caramelised onions actually take?

Thirty to forty-five minutes on a low to medium heat. The ten-minute versions in recipes produce softened, lightly coloured onions, which taste quite different.

Does adding sugar or bicarbonate speed it up?

Sugar adds sweetness but not the depth of real caramelisation. Bicarbonate does speed browning but can give a slightly soapy taste and a mushy texture.