1. Peel and deseed the butternut squash and remove all fibres. Chop the flesh into bite-sized pieces.
2. Peel the potato and cut it into small cubes. Wash and trim the leek and chop it finely.
3. Warm the olive oil in a casserole-type pan and put all the finely chopped vegetables in. Stir fry them for 3-4 minutes, then add the meat stock and nutmeg and simmer for 30 minutes, until the vegetables are soft.
4. Put the soup in the blender and pulse until creamy. Season to taste and simmer for another minute.
5. Whisk the crème fraîche in the mixer until it thickens. Add the yoghurt, salt, pepper and chives.
6. Divide the soup into 6 plates. Garnish each plate with a heaped spoonful of yoghurt mousse and serve hot.