Put the flours and semolina in a large bowl and mix together.
Add the buttermilk, melted butter, onion, chilies, and cumin seeds and stir to combine.
Season with salt then set aside for 20 minutes.
Put the mixture in a blender or food processor and process, until very fine, smooth and fluffy, adding water as required to make a batter.
Transfer the pureed mixture to a clean bowl and allow to stand for 12 hours.
If the mixture thickens a little on standing, stir through a little water to thin it out.
Oil a tawa or heavy-based frying pan (skillet) and heat it.
When hot, add 1 teaspoon oil and pour in a ladleful of the dosa batter, spreading it all around like a thin pancake.
Reduce the heat slightly and fry for 2 minutes on both sides, or until the dosa becomes crisp around the edges and golden brown.
If the dosa is sticking to the pan, pour a little amount of oil around the edges.
Fold 2 sides inwards and transfer to a serving plate. The dosas must be served hot with Coconut Chutney and piping hot Sambhar.
Make the others in the same way.