Why Caramelised Onions Take So Long

Real caramelisation requires the onions’ natural sugars to slowly convert to complex, sweet, deeply flavoured compounds through low, gentle heat over time. High heat burns the outside before the sugars can transform — producing bitter, scorched onions rather than sweet, jammy ones. Any recipe claiming 10-minute caramelised onions is either using high heat (producing inferior results) or describing softened onions, not truly caramelised ones.

How to Make Caramelised Onions

  • 3 large onions (yellow or brown — sweeter than white)
  • 2 tablespoons butter or olive oil (or a mix)
  • Pinch of salt
  • Optional: 1 tablespoon balsamic vinegar or a pinch of sugar (added at the end)
  1. 1

    Slice onions thinly

    Halve the onions and slice into thin half-moons, approximately 3–5mm thick. Thinner slices caramelise more evenly and faster than thick chunks.

  2. 2

    Start in a wide pan over medium heat

    Melt butter or heat oil in a wide, heavy-bottomed pan (a wider pan means more surface area and faster moisture evaporation). Add all the onions — they will look like an enormous pile but reduce dramatically. Add the salt. Toss to coat in the fat.

  3. 3

    Reduce to low-medium and cook 45–60 minutes, stirring occasionally

    After 5 minutes, reduce heat to low-medium. Stir every 5–10 minutes — not constantly. At 15 minutes: significantly reduced in volume, translucent, starting to soften. At 30 minutes: golden and beginning to darken. At 45–60 minutes: deep golden-brown, jammy, sweet, dramatically reduced in volume. If the pan looks dry at any point, add a splash of water to prevent burning.

  4. 4

    Finish with balsamic or sugar (optional)

    In the last 5 minutes, add a tablespoon of balsamic vinegar or a pinch of sugar to deepen the flavour and colour. Cook until the liquid is absorbed.

Uses for caramelised onionsFrench onion soup, pizza and flatbread topping, burger topping, tart and quiche filling, stirred through mashed potato, on a cheese board, in grilled cheese sandwiches, with steak or sausages. Make a large batch and refrigerate for up to a week — they improve if anything as flavours deepen.

Frequently Asked Questions

Slightly. A pinch of baking soda raises the pH and accelerates the Maillard reaction, cutting time to around 30 minutes. The flavour is slightly different (can have a soapy note if overused) — use very sparingly. Covering the pan for the first 15 minutes traps steam and softens onions faster before you uncover and let them caramelise. But there is no genuine shortcut to deep, complex caramelisation — it requires time at low heat.
Heat is too high, or the pan is too dry. Reduce the heat immediately and add a splash of water to deglaze any dark bits stuck to the pan (these are flavour — scrape them up and stir them in). If onions are consistently sticking, add a little more butter or oil. A heavy-bottomed pan (cast iron, stainless steel) distributes heat more evenly and is less prone to hot spots that cause burning.