Rescues · TO SWEAT
The onions browned.
Why it happens
Heat too high, or too little fat. Once they colour they cannot be uncoloured, and the dish will taste of it.
Recovering it
Lightly coloured onions are usually fine — carry on, and know the dish will be a shade deeper than intended. Properly browned ones should be started again if the dish depends on being pale, which is a genuine cost of a few minutes and worth paying in a béchamel or a risotto. Undercooked aromatics can simply be given more time, which is the easiest correction in cooking and the one people are least willing to make.
The mechanism
Sweating and sautéing use the same pan and differ only in temperature and intent. Below browning temperature, an onion's cell walls break down and it releases water and turns translucent; its harsh sulphur compounds volatilise off and its sugars stay unbrowned, so what you get is sweetness and depth without any roasted note. That distinction matters because browned onion tastes of browned onion — assertive and slightly bitter — and there are dishes where that would dominate everything else. A béchamel, a risotto base, a pale soup all want the aromatic foundation and none of the colour. The released moisture is also doing work: it keeps the pan temperature at or below 100°C on its own, which is why a lid or a pinch of salt makes sweating more reliable than watching the heat.
The full TO SWEAT lesson — how to tell, where to practise it, what it carries to.