Method

• 1.
  Place a large sieve over a bowl, ensuring it doesn’t touch the base. Line with muslin or a clean Chux, leaving enough overhanging the sides to cover. Place yoghurt in sieve, draw muslin over the top and chill for at least 1 hour to remove some of the whey and create what the Afghanis call chaka.
• 2.
  Meanwhile, heat oil and half the butter in a heavy-based saucepan over low heat. Add onion and cook with star anise, stirring occasionally, for 15 minutes, then add leek and spring onion and cook, stirring, for a further 25 minutes or until onion is a lovely golden brown. Remove star anise and keep the onion mixture warm.

• 3.
  Mix the flour with a little stock to form a paste, then mix into the remaining stock- this will help thicken the soup and also stop it from splitting.
• 4.
  Whisk the yoghurt and the floured vegetable stock together and stir in the dried mint.
• 5.
  Pour the yoghurt mixture into a clean saucepan. Gently warm the soup over low heat and bring to a gentle simmer, but do not let it boil or it may split. Stir until slightly thickened and season well.
• 6.
  Meanwhile, in a small pan, warm remaining 40g butter with the turmeric and curry powder over medium heat until it foams. Immediately remove from heat and set aside.
• 7.
  Drain the currants and pat dry. To serve, divide the onion mixture between four warm serving bowls and top with currants, walnuts, dill and mint. Pour the warm yoghurt soup into the bowls and drizzle with a little of the warm curry butter.