Watercress soup with labneh and spiced oil

Method

1. To make the labneh, put the yogurt and a pinch of salt in a large double-thickness square of muslin and tie the corners together with string. Loop the string over a wooden spoon handle, then balance the spoon over a large mixing bowl in the fridge so the bag is suspended with a good gap below. Chill for at least 12 hours (see make ahead) – the liquid from the yogurt will drip into the bowl. When the yogurt has the consistency of cream cheese, remove it from the bag and keep chilled in an airtight container.
2. Heat the butter and olive oil in a heavy-based heatproof casserole or large pan over a medium-low heat. When the butter is foaming, add the onion, leeks and garlic and gently cook for 5 minutes or until soft, stirring occasionally.
3. Add the potatoes and stock, then bring to a simmer. Cover and cook for 15 minutes or until the potatoes are tender.
4. Meanwhile, make the spiced oil. Gently warm the spices in a small frying pan over a medium heat for 1 minute, then add the extra-virgin olive oil and cook for 1 minute or until fragrant. Set aside.
5. Add 100g of the watercress to the soup and cook for 2 minutes. Remove from the heat, then blend with a stick blender, adding the remaining watercress a handful at a time as you do so (see tip). Taste and season; add a little lemon juice if needed.
6. To serve, ladle into warmed bowls, then add a dollop of labneh to each and swirl over a little spiced oil, making sure you have a mixture of the spices. Drizzle the remaining spiced oil over the rest of the labneh for people to help themselves.