Boil a medium sized pot of water. Break up the vermicelli and set aside. Put the coconut in a small bowl with a pinch of salt, cover with 150 ml of boiling water and set aside.
Heat the oil in a large nonstick saucepan over a medium heat, then add the mustard seeds. As soon as they start popping, add the chillies, cumin seeds and curry leaves, if using, and stir-fry for 20-25 seconds. Add the spring onions and stir-fry for a further 2 minutes.
Stir in the noodles, 450 ml of boiling water, the coconut and its soaking water, then increase the heat to high. As soon as the water returns to the boil, reduce the heat to very low, cover the pan and cook for 10-12 minutes, stirring occasionally, until all the water has been absorbed.
Spoon the noodles into bowls and top with crushed papad.