First you make filling for Dosa . Add oil in frying pan , make it hot then add mustard seed in it, when it splutter add curry eaves first then chopped onions, green chillies , potato , turmeric powder and salt . Fry for2-3 min. Then add 1/2 cup of water and boil for 2-3 min. Keep aside this masala curry . Mix all ingredients of batter add water to make a medium viscosity. ( After making this batter you can store it for 3-4 days in fridge) Add oil in hot frying pan, add batter on it, spread over on frying pan in low flame . After 2 min. put masala curry on the middle of dosa in horizontally. Fold dosa from two sides .Fry dosa on both sides, add little oil ( If necessary ) . Serve hot with coconut chutney or without chutney .