Lightly oil a 30 x 30 cm (12 x 12 inch) baking sheet. In a bowl, whisk together the flour, yogurt, salt, turmeric, asafoetida, chilli powder and measured water. Pour the mixture into a heavy-based saucepan and bring to the boil, stirring occasionally.
Reduce the heat and cook, stirring frequently, for 10–15 minutes or until thick. Remove from the heat and spoon the mixture on to the prepared baking sheet, spreading it thinly over the surface with the back of a wide spoon. Leave for 5–10 minutes to cool, then cut into 2.5 cm (1 inch) strips and roll them up. Divide between 4 plates.
Heat the remaining oil in a small pan and add the curry leaves and mustard seeds. As soon as the seeds start to pop, spoon the oil over the rolls. Sprinkle over the coconut and coriander and serve hot or cold.