Onions become almost magical when they are allowed to brown properly. At first they soften, then sweeten, and if the heat is right, they can pick up those dark, sticky edges that make them taste fuller and more aromatic. Slightly burnt onions are not the same as charred onions in a bad way. They are richer, deeper and more savoury.
That browning matters because onions have a lot of natural sugar, and heat turns that sugar into a more intense, almost jammy flavour. The darker bits bring complexity to burgers, curries, dals, stews and sandwiches. Without that hint of burn, onions can taste flat. With it, they taste as though they have spent time becoming themselves.