Mix the rice flour and rava with some water and make it into a smooth batter. Add the asafetida, cumin seeds and salt. Cover and keep it in a warm place for about seven hours to ferment.
Mix the chopped onions, green chilies, ginger, cashew nuts and black pepper in a bowl and keep it aside.
Heat a flat pan (Thava) and Grease it with a spoon of oil. Add some water in the dosa batter and make it into a thin batter. Pour 1/2 cup of the batter in center of the pan (tava) and quickly spread with outward spiral motion to form a thin pancake of about 8 inches in diameter. Sprinkle some onion- green chili mixture on top of it. Dot the edges with little oil and cook until golden brown. Using a spatula carefully fold the dosa and remove from the pan. Repeat doing this with the remaining batter to make more dosas. Serve hot with chutneys or sambar.