In a large bowl, whisk together one cup thick yogurt (preferably homemade or Greek yogurt) with water.
Keep aside.
In a mortar, mash together a small bit of fresh ginger root, one small green chilli and some fresh coriander leaves with some sea salt or rock salt.
Add this coarse paste to the buttermilk.
In a small saucepan, heat 1 tsp oil, and add mustard seeds, a few curry leaves and a pinch of asafoetida.
Stir till the seeds splutter and transfer it into the buttermilk bowl.
Mix everything well.
Check for salt. Serve chilled.