Soak rice and urad dal in water overnight. Blend into a smooth batter the next day.
Cook onions, green chillies, and spices like turmeric and chilli powder. Add mashed potatoes or vegetables of your choice and mix well.
Add a little soya sauce, vinegar and salt to the filling for extra flavour and let it cook for two minutes.
Heat a pan, pour the batter, and start spreading it outward in a circular motion into a thin layer. Cook until crispy on both sides.
When the dosa is half-cooked, add the spicy filling, roll it up like a spring roll, and cook until golden brown, serve hot with chutney or sambar.