Wash the whole green moong dal and idli rice together in several changes of water until the water runs mostly clear, then drain.

Soak the washed moong dal and rice together in enough water to cover by 2 inches until the dal softens and splits easily when pressed.

Drain the soaked dal and rice, then grind with the ginger, green chilies, cumin seeds and just enough fresh water to a smooth, thick, pouring-consistency batter.

Stir the salt into the freshly ground batter and adjust the consistency with a little water so it spreads easily but still holds together.

Heat a flat tawa over medium-high heat and test readiness by sprinkling a few drops of water on it — they should sizzle and evaporate immediately.

Pour a ladleful of batter onto the center of the hot tawa and spread it outward in a thin, even circle using the back of the ladle, working from the center out.

Scatter a small handful of finely chopped onion evenly over the surface of the spreading batter and press down gently with the back of the ladle.

Drizzle oil around the edges and over the top, then cook uncovered until the underside turns golden brown and crisp and the edges begin to lift on their own.

Flip the dosa carefully and cook the other side briefly until it develops light golden spots.

Fold the dosa in half or roll it, remove from the tawa, and repeat the pouring and cooking process with the remaining batter.