Almost every Indian gravy starts with onions cooked alone for a long stretch before anything else joins them. The order is not habit, and skipping it produces a fault no amount of extra simmering repairs.
What browning actually changes
Onions carry sugars and sulphur compounds that convert into deeper, rounder flavour only under sustained dry heat. That requires a pan hot enough to drive off surface moisture before colour begins to develop.
As the water leaves, the remaining sugars concentrate and start to caramelise, while sugars and amino acids react separately to create browned aroma compounds. Neither reaction runs at the temperature of a wet pan.
The result is a base that reads as sweet and savoury at once rather than sharp and raw. Everything added afterwards is tasted through it.
Why tomatoes stop the process
Tomatoes are mostly water and carry a substantial acid load. Adding them pulls the pan down to around boiling point, and browning simply stops at that ceiling.
Acid also stiffens the pectin that holds plant cell walls together. Onion pieces that meet acid before they have softened stay firm for the remainder of the cooking time.
This is why a cook who tips both in together finds crunchy onion in the finished dish an hour later. The window for softening has closed.
How the fat carries the flavour forward
Browning happens in oil or ghee, and many of the aroma compounds released are fat-soluble rather than water-soluble. The fat becomes the medium that spreads them evenly through the gravy.
When the mixture has cooked far enough, fat separates and pools at the edge of the pan. Cooks read that separation as the signal that the base is finished.
A base that never reaches this point tastes thin, and the spices sit on top of the dish instead of through it.
Why the ginger and garlic sit between the two
Ginger and garlic paste usually goes in after the onion has coloured but before the tomato arrives. Both burn far faster than onion because they are cut fine and contain less bulk water.
Cooking them briefly in hot fat removes the raw sulphurous edge without scorching. A few minutes is normally enough, and the smell shifts noticeably when it is done.
Adding them at the start means they burn during the long onion stage, which leaves a bitterness that carries into the finished plate.
What changes if the step is shortened
Rushing the onion produces a gravy that tastes of its individual parts rather than of one thing. Tomato reads as sour, spice reads as dusty, and nothing binds.
Lengthening it in the other direction has limits too, since onion taken past deep brown turns bitter and the sugars begin to burn rather than caramelise.
The practical target sits between those two failures, which is why recipes describe the colour and the smell rather than giving a fixed time.