Meanwhile, temper the spices. Put the oil in a wok or heavy-based pan over a medium-high heat and, once hot, toss in the seeds and the curry leaves. When they start to sizzle, add the onion and saute for five minutes, stirring to coat in the spices. Add the ginger, garlic and green chillies, and cook for five more minutes. Now add the coriander, chilli powder and turmeric and stir well for a couple of minutes. If the spice mixture starts to stick, add a few tablespoons of hot water and scrape it off the base of the pan. If your peanut butter is a little stiff, give it a mix with some of the coconut milk in a hand blender, then add to the pan with the rest of the coconut milk and the tahini. Cook for five minutes at a high simmer, then stir in the tamarind paste and add salt to taste.