Heat the oil in a large saucepan. Add the onion and garlic, then cook for 5 minutes or until the onion is softened but not browned. Add the celeriac. Pour in the boiling water and stir in the stock cube. Bring to the boil over a high heat, then reduce the heat and cover the pan. Cook the soup gently for 10 minutes or until the celeriac is tender.
Add the spinach to the soup and stir well. Increase the heat and bring the soup to the boil, then remove the pan from the heat. Leave the soup to cool slightly before puréeing it, in batches, in a blender or food processor until smooth. Alternatively, you can purée it in the pan using a hand-held blender. The soup will be fairly thick.
Reheat the soup, if necessary, then stir in a little grated nutmeg, salt and pepper to taste. Ladle the soup into warm bowls. Swirl a spoonful of yogurt into each portion, garnish with fresh chives, then serve at once.