Steps
1. Soak rice and black gram separately in water for 4 hours. Then wash with water several times.
2. Grind black gram in a wet grinder with 1/4 cup water. Grind rice in the same manner. Mix both flours, then add a little salt. Allow the mixture to ferment for 12 hours.
3. After fermentation it is ready for making dosai.
4. Heat a nonstick pan over medium flame. When the pan is hot, brush it with vegetable oil. Spread 1/2 cup for the mixture on it, then cover it with a lid. After 2 mn, collect the dosai
5. Preparation for Masala Dosai:
6. Heat oil; add onion and saute for a minute or until transparent. Add tomatoes; stir for a minute
7. Add potato, carrot, salt and pepper. Cover and cook on low heat for 10 mn.
8. Spread 1 tbsp of this mixture in the center of a dosai. Roll and serve with coconut and sambhar.

Source
International Rice Institute (IRRI) & International Women's Organization (SUHAY), VIRMANI, Inderjeet (ED)(1991), Home Chefs of the World - Rice and Rice-based Recipes, ISBN 971-22-0023-X, Manila, Philippines