Onion Dosa is the South Indian griddle dosa with chopped onions scattered onto its surface while it cooks, so they half-fry into the crepe rather than sitting on top as a garnish. The angle is the integration. This is not a dosa served with onions alongside; the onion becomes part of the disc itself, sweetening and browning into the batter as it crisps, which changes both the flavor and the texture of every bite. It is one of the standard ways a tiffin cook personalizes a plain dosa without changing the underlying batter at all.
The build runs on a fermented batter of soaked rice and urad dal, ground and left to rise overnight so it develops the tang and the bubble structure that lets it crisp. A flat tawa is heated until a flick of water dances and dies, then wiped down, and a ladle of batter is spread from the center outward in a quick widening spiral into a thin even round. While the underside is still setting, a generous handful of finely chopped raw onion, often with green chili and a little coriander, is scattered across the wet top and pressed down lightly with the back of the ladle so it embeds into the surface. Ghee or oil goes around the edge, and the dosa is left undisturbed until it lifts clean with a deep gold, lacy underside and the onion gone soft and brown where it met the heat. Good execution gives a crisp dosa with onions cooked through and lightly caramelized into the crumb. Sloppy execution is onion piled on so thick and so late that it stays raw and slides off, or a batter spread too thick so the center stays pale and soft while the onion never gets near enough heat to sweeten.
Variation is mostly a matter of fold and accompaniment. It can be served open and flat, folded into a half-moon, or rolled into a tube, almost always with coconut chutney and sambar. A potato masala can be tucked inside to make it a masala-and-onion combination, and a smear of red garlic chutney across the inside is common in the Karnataka style. The semolina-based rava dosa takes onions in a completely different, lacier batter and deserves its own article rather than being crowded in here, as does the thick, soft uttapam, which treats onion as a pressed-in topping on a pancake rather than a crisp griddle disc. Onion Dosa, specifically, is judged on one thing: whether the onion has actually cooked into the crepe and gone sweet, or is just sitting there raw.