Cooks across South Asia fry a large batch of sliced onions and keep them for weeks. The frying is a preservation step as much as a flavor one.
Frying removes almost all the water
A raw onion is mostly water, and frying drives that off until the slices are brittle rather than limp. What remains is concentrated sugar, sulfur compounds and browned solids.
Once the moisture is gone, the material is stable in a way the raw vegetable never is, for the same reason that dried foods keep and fresh ones do not.
The browning also builds flavor that raw onion does not have, since sugars and amino acids react at frying temperatures into deeply savory compounds.
The slow step is moved out of the meal
Softening and browning onions properly is the longest stage of many gravies and cannot be rushed without scorching. A stored batch removes it entirely.
A weeknight dish then becomes a matter of blooming spices, adding the fried onions with liquid, and simmering, which changes the time profile of the whole meal.
Because a large batch fries almost as fast as a small one, the cost of preparing several weeks' worth is a single evening rather than a repeated tax.
They dissolve back into a gravy
Ground or crushed and added with liquid, fried onions rehydrate and disperse, thickening the sauce and coloring it deeply.
They do not behave identically to freshly cooked onion, being darker, sweeter and slightly bitter at the edges, which is why some dishes call for them specifically.
Whole fried slices are also used as a garnish, where the point is the crisp texture rather than the dissolved body, so cooks reserve some uncrushed.
The technique that decides the result
Slices must be thin and even, since thick pieces still hold water when thin ones have burned. A mandoline does this far better than a knife.
Onions are removed from the oil while still lighter than the target color, because the residual heat keeps browning them on the draining tray.
Draining thoroughly matters for storage, as retained oil goes rancid and is what usually spoils a jar before the onions themselves fail.
Storing and buying
An airtight jar in a cool cupboard suits a few weeks, and freezing extends it, since the enemy is absorbed humidity that softens the slices back toward limpness.
Indian and Middle Eastern groceries also sell them ready-fried in bags, which saves the mess but varies in oil quality, so the frying medium is worth checking on the label.
The crisp-fried onions sold in American supermarkets for casseroles are a different product, since they are battered in flour and behave like a coating rather than dissolving into a gravy.