Place semolina, rice flour, all purpose (plain) flour, onion, yogurt, cumin seeds, minced ginger, green chilies, coriander (cilantro) leaves, curry leaves, salt and water in a large bowl. Stir to combine. Cover and set aside for 15 minutes.
Place a crepe pan or frying pan over medium heat.
Brush the pan lightly with oil.
Stir the batter.
Pour ¼ cup batter into the pan in a very thin layer.
The lace-like pattern will appear on the dosa immediately.
Drizzle with a little ghee.
Cook until the top of the dosa is no longer wet and bottom has turned light brown.
Run a spatula around the edge of the pan to loosen the dosa.
Fold and transfer to a serving plate.
Repeat with the remaining batter. Serve hot with sambar and chutneys.