One of the most fundamental steps that decides the taste of a dish is toasting the onion. Even if you cook the same amount of onion in the same oil at the same heat, the result can be completely different. The onion you chop for menemen does not want the same time and heat as the onion you chop for onion soup.
In Turkish cuisine, the first step of nearly every dish is to toast the onion. As ordinary as this move looks, it decides the direction of the recipe: in a meat dish where you build a deep, dark base, the technique is one thing; in a vegetable sauté where you only want to imply the presence of onion, it is another. This guide explains how to take the onion to a golden color, which technique fits which recipe, and how to avoid the three most common mistakes.
Why is it so important?
As the onion heats up, the sulfur compounds inside it break down and sugar molecules come to the surface. Through this Maillard reaction a sweet, caramel taste forms. At too-high heat the sugar burns and turns bitter before the onion browns; at too-low heat it takes too long to take color and the onion stays as if boiled.
The summary of the science: in raw onion the alliinase enzyme, when the cell ruptures, combines sulfur compounds and produces propanethial-S-oxide (the compound that brings tears to the eye). Heat inactivates this enzyme, the structure of the sulfur compounds changes, and sweet volatiles like form. After the temperature passes 140°C the dehydration of sugar begins; at 160°C caramelization, at 170-180°C Maillard fully kicks in. The three-layer "sweet + umami + slightly nutty" taste profile of a toasted onion comes from here.