Put the buttermilk into a large pan, add the rice flour, turmeric and some curry leaves, then season with salt. Set aside.
Heat 1 tablespoon oil in a frying pan (skillet) over medium heat, add the coriander seeds, dal, asafoetida and dried red chillies and stir-fry for about 1 minute, or until the seeds start to splutter and the chillies turn a shade darker.
Transfer to a small blender or food processor, add the coconut and green chillies and process, adding a little water if necessary, to make a smooth paste.
Blend this paste into the buttermilk mixture and bring to a boil. Cook for 10 minutes over medium heat.
To make the tempering, heat the remaining oil in a small frying pan over medium heat, add the mustard seeds and stir-fry for about 1 minute, or until they start to splutter, then pour over the curry.