Soak the moong dal in a bowl of water for 2-3 hours.
Drain the dal, reserving 250ml (8fl oz / 1 cup) of the water, and transfer the dal to a blender or food processor.
Add the ginger and chillies and season with salt, then process to make a paste.
Put the paste in a bowl and mix in enough of the reserved water to make a thick batter.
Put the sliced onion in a separate bowl, add the ground cumin and chopped coriander and set aside.
Heat a frying pan (skillet) and, when hot, pour in 1 teaspoon of the oil.
Pour 1 ladleful of the batter into the frying pan and spread it out like a thin pancake, then pour 1/2 teaspoon of the oil around the edges and cook over medium heat for about 2 minutes, or until golden brown.
Sprinkle some of the chopped onion mixture in the middle of the pancake and spread it around.
When the pancake is well cooked, add a little more oil and fry for a further 1 minute, or until the edges are crispy.
Fold the pancake in half, remove from the pan and keepwarm in a covered dish.
Make the others in the sameway. Serve hot with Sambhar and any pickle, or Mintor Raw Tamarind Chutney.