-
1
Heat the oil in a heavy pan over medium heat, then add the mustard seeds and urad dal together so the dal toasts in the same aromatic fat the seeds are already flavouring. Cook until the mustard seeds pop and the dal turns pale gold — the sound of active popping subsiding is the signal to proceed, because seeds still popping will continue to colour and can turn bitter in moments.
-
2
Add the curry leaves — they will sputter immediately as their moisture hits the hot oil, releasing the volatile fragrance that defines the base — then the green chilli and onion. Cook, stirring, until the onion is translucent and soft, with no remaining raw sharpness; the pan will smell of sweetened allium rather than raw onion.
-
3
Add the turmeric and stir to coat the aromatics evenly, so it blooms in the residual fat rather than sitting raw in the finished filling; the contents of the pan should turn uniformly bright yellow within thirty seconds. Add the crumbled potato and mix through, applying pressure with the back of a spoon to break larger pieces — the target is a rough, cohesive crumble: visible potato pieces, no smooth paste. Season with salt, remove from the heat, and allow to cool slightly, because a filling that is still steaming will soften the dosa before it has a chance to crisp.
-
4
This is where the masala dosa lives or dies. Heat the tawa over medium-high heat until a few drops of water evaporate on contact, then pour one portion of dosa batter onto the centre and spread immediately in a thin, outward spiral so the batter moves before it can seize on the hot surface. When the surface begins to lose its wet sheen and the underside is setting — but before the dosa has fully crisped — place one quarter of the potato filling across one half of the dosa, because a fully crisped dosa will crack on the fold, while an under-set one will tear when you try to move it.
-
5
Apply ghee around the exposed edges and perimeter of the dosa with a spoon so the fat reaches the parts of the dosa that had no direct contact with the tawa's surface oil. The edges will deepen to golden-brown and pull slightly from the tawa surface, giving the characteristic crisp, lacy border that no amount of extra cooking time without fat can replicate.
-
6
Fold the unfilled half of the dosa over the filled half and slide onto a plate immediately so the residual heat of the tawa does not overcook the underside while you hesitate. The exterior should be audibly crisp when tapped lightly — a hollow, papery sound — and the filling, warmed through by steam during the final minute on the tawa, will be hot at the centre. Serve at once with sambar and coconut chutney alongside.