Onions take far longer to caramelise than any recipe admits

The points below about caramelising onions are ordered by how much difference they make, not by how often they get repeated.

Three stages, not one

Onions first sweat, releasing water and turning translucent, which takes several minutes and involves no colour. Then the surface begins to brown through Maillard reactions between sugars and amino acids, which is where most of the savoury flavour comes from. Only later does true caramelisation of the sugars themselves contribute, giving the deep sweetness of a properly slow batch.

Recipes that say ten minutes are describing the first stage and borrowing the vocabulary of the third.

Water is the rate limiter

Onions are mostly water, and none of the browning can start until enough of it has evaporated. A crowded pan holds that water in and keeps everything at boiling point, which is below the browning threshold. This is why a wide pan with a large surface area beats a deep one, and why a lid at the start then off is a common trick.

Salt early draws water out faster, which helps the sweating stage and then gets out of the way.

Heat is a trade, not a shortcut

High heat browns the edges before the interiors have softened, giving scorched flecks in an undercooked mass. Low heat gives even colour throughout but genuinely takes a long time, often three or four times the stated figure. Medium heat with occasional stirring, then lower once colour starts, is the practical compromise most cooks land on.

On a weeknight, a splash of water when the pan starts to catch dissolves the fond and redistributes it, which speeds the appearance of colour.

What the shortcuts actually do

A pinch of bicarbonate of soda raises the pH and accelerates browning markedly, at the cost of a softer, sometimes slippery texture and a slightly different flavour. Added sugar produces colour quickly but is caramelising the sugar you added rather than developing the onion. Both are legitimate if you understand the trade, and neither produces the same result as time.

The part that matters: the honest shortcut is to make a large batch when you have the time and freeze it in portions.

Cut for the outcome you want

Slices with the grain, from root to tip, hold their shape and stay distinct. Slices across the grain break down faster into a jammy mass, which suits a sauce base or a soup.

The part that matters: even thickness matters more than the direction, because thin pieces burn while thick ones are still raw. Onion variety matters too: sweeter varieties have more sugar and colour faster, and store onions are more pungent and take longer.

Ovens, pans and produce vary — trust what you can see and hear over any timing.

What it is for

Deeply cooked onions are a concentrated savoury base, which is why so many cuisines start there. They add sweetness and body to soups, sauces and braises without adding any sugar you can identify. A jar in the fridge or a tray in the freezer turns a plain omelette, a sandwich or a bowl of pasta into something with depth.

Treat them as a pantry item you make occasionally rather than a step you rush every time.

Everything above, in order of what to do first

1. Three stages, not one. Onions first sweat, releasing water and turning translucent, which takes several minutes and involves no colour.
2. Water is the rate limiter. Onions are mostly water, and none of the browning can start until enough of it has evaporated.
3. Heat is a trade, not a shortcut. High heat browns the edges before the interiors have softened, giving scorched flecks in an undercooked mass.
4. What the shortcuts actually do. A pinch of bicarbonate of soda raises the pH and accelerates browning markedly, at the cost of a softer, sometimes slippery texture and a slightly different flavour.
5. Cut for the outcome you want. Slices with the grain, from root to tip, hold their shape and stay distinct.
6. What it is for. Deeply cooked onions are a concentrated savoury base, which is why so many cuisines start there.

The takeaway

Wide pan, moderate heat, and make far more than you need while you are standing there.

Taste it before it leaves the pan. That is most of cooking.