Caramelised Onion

01Ingredients

1000g onions, thinly sliced
3 tbsp olive oil or butter
1 tsp salt
1 tbsp brown sugar (optional)
2 tbsp balsamic vinegar (optional)

02Method

Heat oil in a large, heavy-based pan over medium-low heat. Add sliced onions and salt.
Cook, stirring occasionally, for 40-45 minutes until deeply golden and soft - resist the urge to rush this with higher heat.
If the pan gets dry or onions start to catch, add a splash of water and scrape up any browned bits.
Stir in brown sugar and balsamic vinegar if using, and cook a further 5 minutes.
Use immediately, or cool and store.

03Notes & substitutions

Red or brown onions both work well - red onions give a slightly sweeter, milder result.
Butter gives a richer flavour than olive oil, though either works well.
Add a sprig of fresh thyme in the last 10 minutes for a flavour lift.
Vegetarian and vegan as written (without butter).

04Common mistakes

Cooking too hot — high heat browns the outside before the onion softens through, giving a burnt-but-raw result instead of caramelisation.
Not stirring enough — onions at the bottom of the pan catch and burn if left too long between stirs, especially in the last 15 minutes.
Giving up too early — proper caramelisation looks unpromising for the first 20 minutes before it suddenly turns; don't mistake this stage for it not working.

05Storage & leftovers

Fridge

Airtight container, up to 1 week.

Freezer

Freezes well in small portions, for up to 3 months.

Serving

Stir through pasta, pile onto a sandwich, or use as a base for French onion soup.

More About This Recipe

06Ingredient notes

Onions. A larger 1kg batch is worth doing at once, since the process takes the same amount of attention regardless of quantity, within reason.

Salt. Added early, it helps draw moisture out of the onions, which speeds up the softening stage before caramelisation begins.

Balsamic vinegar. Optional, but it adds acidity that balances the sweetness - worth including if you have it.

07Troubleshooting

Onions are burning, not caramelising. Heat is too high - drop to medium-low and be patient; there's no way to safely speed this up.
Onions are taking much longer than 45 minutes. The pan may be overcrowded, or the heat too low - a wider pan and slightly higher (but still gentle) heat should help.
Onions taste flat despite looking caramelised. Needs more salt, or didn't cook quite long enough - caramelisation should taste deeply sweet, not just look brown.
Pan is sticking badly. Add a splash of water and scrape up the browned bits - this is where a lot of the flavour lives, not something to avoid.

Frequently Asked Questions

Why does caramelising onions take so long?

Caramelisation is a slow chemical reaction between the onion's natural sugars and heat. Turning up the heat to rush it browns and burns the onions rather than caramelising them - there's no shortcut.

Can I freeze caramelised onion?

Yes - it freezes well for up to 3 months, and is worth making in a large batch specifically to freeze in smaller portions.

Why are my onions burning instead of caramelising?

The heat is too high. Drop it to medium-low, and expect the process to take the full 40-45 minutes rather than trying to speed it up.

Can I make a bigger batch?

Yes, though a larger volume of onions needs a wider pan or a second batch - overcrowding a single pan steams the onions rather than letting them caramelise.