This is the first lesson in Cook Like a Chef, a short series on the things a chef does without thinking and nobody bothers to explain. I am starting with onions, because nearly every savoury recipe on this site starts with them, and the way you cook them decides how the dish tastes.
On my Japanese potato curry I say that if it tastes flat, the answer is almost always the onions rather than the spice. That goes for most stews, soups and sauces. Get the onions right and half the work is done.
Three Different Jobs
Recipes say “fry the onions” as if it were one thing. It is three, and they give three different results.
| Heat | Time | Colour | What it gives | |
|---|---|---|---|---|
| Sweat | Low to medium | 8 to 10 minutes | None | A soft, sweet base that sits quietly under everything else |
| Soften to gold | Medium | 10 to 15 minutes | Light gold at the edges | More sweetness and a little roasted depth |
| Caramelise | Low | 45 minutes to an hour | Deep brown all through | Jammy and very sweet, a flavour in its own right |
The times are a guide, because a wide pan, a full pan and a hot hob all change them. Go by how the onions look and smell, not the clock.
What Is Happening in the Pan
A raw onion is about nine tenths water. When you cut it, it makes sharp, sulphury compounds to put animals off eating it, which is what stings your eyes and what tastes harsh when an onion is raw. Heat turns those into the savoury, almost meaty flavour that a cooked onion adds to a dish.
Onions also store their energy as chains of sugar, and slow cooking breaks those down, which is where the sweetness comes from. That sugar is mostly fructose, and fructose starts to colour at just above the temperature of boiling water, far lower than ordinary sugar does. That is why onions brown so easily, and why they catch so easily too.
While the onions are still wet, the water in them holds them at about the boiling point, so they soften without colouring. Once most of that water has gone, they colour quickly. Low heat keeps you in the softening stage for longer; high heat drives the water off and goes straight to browning. If you want to know more about browning itself, I explain it in my article on the Maillard reaction.
How to Sweat Onions
- Cut them small and even. Pieces of the same size cook at the same speed, so none of them colour while the rest are still raw.
- Use a little fat and a medium heat. Enough oil or butter to coat the onions, and put them in before the pan is very hot.
- Stir now and then. Not constantly, just enough that nothing sits still against the hottest part of the pan.
- Stop when they are soft and see-through. The raw smell will have gone and a piece will squash easily against the side of the pan.
- If they start to colour, turn the heat down or add a spoonful of water. The water pulls them back to boiling point and buys you time.
This is the base for most soups and Polish dishes. On my leczo the onion gets five or six minutes until it is soft and translucent, and on my lazanki the onions get five minutes until they are translucent before the mushrooms go in.
Softening to Gold
Most curries and stews want the onions taken a little further. Sweat them first until they are soft and translucent, then let them sit a little longer on a medium heat until the edges turn light gold. Some gold is what you want, not a deep brown. On the Japanese curry, taking them all the way pushes the whole dish towards a darker, roastier flavour than it should have.
Do not skip the soft stage to save time. Onions that colour while they are still crunchy taste scorched on the outside and raw in the middle, and that sharpness carries through the whole pot.
How to Caramelise Onions Properly
Caramelised onions (caramelized if you are in the US) take far longer than most recipes admit. Ten minutes on a high heat gives you onions that are browned at the edges and still firm. Proper caramelised onions take the better part of an hour.
- Slice them thinly and use a wide pan, so the water has somewhere to go.
- Start on medium to sweat them, then turn it down to low once they have collapsed.
- Stir every few minutes. As they get darker, stir more often, because the sugar catches faster near the end.
- When a brown layer forms on the bottom of the pan, add a splash of water and scrape it up. That layer is flavour, and stirring it back into the onions is what gives them their deep colour.
- Stop at a deep, even brown. Push past that and they turn bitter.
You can cook a big batch and keep it in the fridge for a few days. It is worth having for burgers, toasties and soups.
Garlic Goes in Later
Garlic has much less water than onion and a lot more sugar, so it dries out and browns much faster. Put it in with the onions at the start and it is burnt and bitter long before the onions are soft. Add it near the end, when the onions are nearly done, and give it a minute. On the leczo the onion gets its time before the garlic goes anywhere near it, for exactly this reason.
Which Onion to Use
- Yellow or brown onions are the everyday ones. They are the strongest and driest, they keep for months, and they are what I mean when a recipe just says onion.
- Sweet onions such as Vidalia are grown in soil with less sulphur, so they are milder rather than sweeter. They are at their best raw.
- Red onions are mild and look good raw, but cooking dulls their colour.
- Shallots are small, finer in texture, a little milder and sweeter.
Cutting Onions Without Tears
Put the onions in iced water for 30 minutes to an hour before you cut them. The cold slows down the reaction that makes the stinging compounds, and the skin turns less brittle, so it peels off more easily too.
Practise it on these
- Japanese potato curry: sweated soft and translucent, then barely golden.
- Leczo: onion softened properly before the garlic goes in.
- Hungarian goulash: the onions are given real time before the paprika goes in.
- Lazanki: onions softened until translucent before the mushrooms go in.
Onion FAQ
How long does it take to sweat onions?
About 8 to 10 minutes on a low to medium heat for chopped onions, until they are soft and see-through with no colour.
What is the difference between sweating and frying onions?
Sweating is slow and low, and the aim is soft onions with no colour. Frying uses more heat and browns them. Sweated onions give a sweet, quiet base; fried onions give more depth and a roasted note.
Why do my onions burn instead of caramelising?
The heat is too high. Once the water has gone, the sugar in the onions colours very fast. Turn the heat down, add a splash of water and scrape the pan, and give it more time.
Can you caramelise onions faster?
A little, by starting on a higher heat while the onions are still wet. Once they start to brown you have to slow down or they burn. There is no real way to get an hour of flavour in ten minutes.
Should onions be translucent or golden?
It depends on the dish. Translucent for soups, light sauces and anything delicate, light gold for curries and stews, deep brown only when the onion is the point of the dish.
Why does my dish taste flat even though I followed the recipe?
Often it is the onions. If they went in with everything else and never had time to soften on their own, the dish is missing its sweet base. For other causes, see my kitchen troubleshooting guide.
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