For the dosa batter:
4 cups raw rice (OR 3 cups raw rice + 1 cup parboiled rice) 
1 cup Urad dal
1/4 cup poha (a big handful)
1 tbsp methi seeds
Salt to taste
Water for soaking
Oil+Ghee (or plain oil) for making the dosas 

For making batter:

Soak the rice, methi seeds, poha and dal together for 3 – 4 hours. Grind until very smooth, adding a little water.
Add salt and mix the batter well. Let it ferment overnight (or 8-10 hours). The batter should have risen and look light & airy. 

For the (potato filling):

2 large potatoes, boiled and mashed
1 large onion, chopped
2-3 green chillies, chopped fine
1 tsp grated ginger (optional) 
1 tbsp chopped coriander
1 tsp lime juice
1 tbsp water 
salt to taste
Tadka: oil, mustard seeds, turmeric

Heat oil in a pan. Add mustard seeds. When they pop, add turmeric, green chillies and onion.
Fry until the onions are translucent.
Add the mashed potatoes, salt and a tbsp of water. Mix well. Finish with lemon juice and chopped cilantro.