Combine and soak the rice, fenugreek seeds and urad dal for 4 hours.
Separately soak the aval for 1 hour.
Grind the aval first and then add the rice mixture.
Grind to a smooth batter.
Add salt and keep aside for 12 hours to ferment.
Add sugar, rava, turmeric powder and mix well.
To make the potato masala, heat oil in a pan.
Add the mustard seeds, curry leaves and asafoetida powder.
Fry for 30 seconds.
Add the onions and saute for a minute or two.
Add the peas, potatoes, green chillies, ginger garlic paste and mix well.
Cook for a minute.
Add cumin powder, turmeric powder, salt and stir well.
After another minute, add coriander leaves and remove from flame.
Heat a nonstick tawa over medium flame.
Pour a ladleful of the batter and spread well to make a thin dosa.
Add oil around the edges and cover with a lid for a minute.
Add 1 or 2 tblsp of the onion chutney to the centre and spread evenly.
Add 2 to 3 tblsp of the potato masala and fold the dosa into a triangle.
Cook for 30 seconds and remove.
Serve hot with coconut chutney and sambar.