Soak the rice and the fenugreek for 12 hours or overnight.
Soak the Urad Dhal for half an hour at the end of the rice soaking period.
Drain the rice and dhal.
Grind in a blender, or food processor with the water. Do it in two batches. I used the Vitamix which grinds very finely.
Blend it really well until frothy, smooth and chalky.
Then put the mixture into a bowl and cover with a tea towel. Do not put it in the fridge unless you live in a hot country like India. It took 3/4 days for the mixture to ferment and I have a warm kitchen. Check it every day. When it's bubbly and tastes a bit sour, it's fermented. If you cook the batter when it isn't sufficiently fermented, the dosa will be heavy and tasteless.
Add the sugar and salt.
Heat the flat pancake pan. If it becomes too hot, cool it down with kitchen roll dipped in cold water.
Add a little ghee or vegetable oil or coconut oil to the pan then put in a ladle of the batter. Work quickly with the ladle skimming the top of the batter, spreading the batter out into a large circle. The dosa should be thin.
Add more ghee to the dosa, sprinkling it around the edges and centre.
The edges will curl up when it's almost cooked, then you must use a very thin flat fish slice to loosen the dosa from the bottom of the pan.
Traditionally you scoop a few spoonfuls of potato curry into the centre.
When the underside is golden, start to fold over one side of the dosa, then the other side.
Serve also with fresh coconut chutney.