1. Melt the butter in a pan.
2. Add in the sweet potatoes, leeks and watercress, making sure all the ingredients are coated with butter.
3. Season with a little black pepper, put the lid on the pan and let the vegetables sweat over a very gentle heat for 20 minutes.
4. Be sure to give a little stir half-way through.
5. Add the stock, replace the lid and let everything simmer for a further 10-15 minutes or until the vegetables are nice and soft.
6. Let the mixture cool a little before transferring to a blender and giving it a good whiz!
7. Once smooth, return to pan and stir in the live yogurt and a little more black pepperto taste.
8. When nice and warm, serve in soup bowls and garnish with watercress leaves and a drizzle of yogurt.