An enormous number of savoury dishes begin with cooking onions. And an enormous number of recipes describe this in a way that guarantees you'll do it badly.
"Fry the onions until golden, about five minutes" is one of the most misleading instructions in cooking.
What's actually happening
Onions cooked slowly undergo several distinct processes.
First, water evaporates. Raw onion is around ninety percent water and the initial phase is largely driving that off. Nothing much happens flavour-wise until it's substantially gone.
Then cell walls break down, releasing sugars that were bound in the structure.
Then those sugars caramelise, and simultaneously Maillard reactions occur between sugars and amino acids, producing hundreds of new aromatic compounds.
The sharp sulphur compounds that make raw onion pungent break down and volatilise across the process.
The end result is something completely different from what you started with — sweet, deep, savoury. That transformation takes time and there's no shortcut that produces the same result.
The stages
Worth being able to identify, since recipes are inconsistent about terminology.
Softened, three to five minutes. Onions have collapsed and turned translucent. No colour. Still tastes recognisably of raw onion.
Golden, ten to fifteen minutes. Pale gold at the edges. Sweetness beginning. This is what most recipes actually mean by golden.
Browned, twenty to thirty minutes. Genuinely brown throughout, substantially reduced in volume, sweet and deep. This is what a great many Indian recipes require and what almost nobody does.
Caramelised, forty-five minutes or more. Deep brown, jammy, dramatically reduced. Used in French onion soup and similar. A different thing again.
Recipes claiming caramelised onions in ten minutes are describing something that isn't caramelised. This has been pointed out repeatedly and the instruction persists.
Getting it right
Medium heat, not high. High heat browns the outside before the inside has softened, producing burnt edges and raw centres. That tastes acrid rather than sweet.
Enough fat. The onions need to be moving in fat, not sticking to dry metal. Skimping here is a common cause of burning.
Salt early. A pinch at the start draws out moisture through osmosis, which speeds the evaporation phase.
Don't stir constantly. Contact with the pan surface is what produces browning. Stirring every couple of minutes is enough — constant stirring prevents colour developing.
Deglaze when it sticks. Brown residue on the pan base is concentrated flavour. A splash of water lifts it back into the onions and prevents burning. Repeat as needed.
Don't crowd. Too many onions in too small a pan means they steam rather than fry. A wide pan, or cook in batches.
How they're cut
Affects the result more than people expect.
Slicing with the grain, from root to tip, produces pieces that hold their shape better. Slicing across the grain produces pieces that break down more readily.
For browning, thinner slices brown faster and more evenly. Chunks take considerably longer and brown unevenly.
For dishes where the onion should disappear into the sauce, finely chopped is right. For dishes where you want distinguishable pieces, sliced.
The varieties
Not interchangeable, though most recipes assume they are.
Red onions are generally sharper raw and less suited to long browning — they can turn a slightly unappealing greyish colour.
White and yellow onions vary in sugar and sulphur content by variety and season. Yellow are the general-purpose choice for cooking.
Shallots are sweeter and more delicate, cook faster, and are worth the extra cost in dishes where they're prominent.
Spring onions and their relatives are a different proposition entirely and shouldn't be substituted into a recipe requiring browned onion.
Doing it in advance
Given that this is the slowest step in a great deal of cooking, doing it once for several meals is the obvious efficiency.
Browned onions refrigerate for several days and freeze for months. Frozen flat in portions, or in ice cube trays, they're ready when needed.
This single practice removes twenty-five minutes from a large proportion of weeknight cooking and is probably the highest-value batch preparation available.
The crying
Since it comes up. The irritant is a volatile compound formed when onion cells are damaged, which reacts with moisture in the eyes.
Things with some basis: chilling the onion first, which reduces volatility. A sharp knife, which damages fewer cells. Good ventilation or a fan drawing air away. Cutting near a flame, which can help by drawing the volatile compounds upward.
Things without much: holding a spoon in your mouth, chewing gum, wearing goggles is effective but you'll look ridiculous, which may be an acceptable trade.
The frying versus sweating distinction
Worth naming because recipes use the terms loosely and they describe genuinely different operations.
Sweating means cooking in fat over low heat with a lid on, so moisture is retained and the onions soften without colouring. Used where you want sweetness and body without the deeper flavours of browning — many soups and stocks.
Frying means cooking uncovered over higher heat so moisture escapes and browning occurs.
Doing one when the recipe wants the other produces a dish that is recognisably wrong without being obviously so. A soup made with browned onions tastes heavier than intended; a curry base made with sweated onions tastes thin.
The giveaway in a recipe is whether a lid is mentioned, and whether the instruction says softened or coloured.