Grind spinach, coconut, green chillies, coriander leaves, ginger, garlic and peppercorns to a smooth paste. Add the ground paste to buttermilk and mix well.
Heat oil in a non-stick pan, add mustard seeds, cumin seeds and curry leaves. When the seeds splutter, add onion and sauté till the onion turns translucent.
Remove from heat and allow it to cool down to room temperature. Add this to the buttermilk mixture with salt and mix well. Serve with steamed rice and a dollop of pure ghee.