Boil and cut (banana) into pieces. Cut into long strips. Cut green chilies into small pieces.
Heat oil in a pot. Add mustard seeds, when they crackle add urad dal, curry leaves, asafetida, turmeric powder and green chilies.
After 2 minutes add and stir till brown. Mix boiled (banana), salt and sugar.
Make plain dosa and spread the masala in the center and serve.