1. Heat the oil in a large saucepan.
2. Add the onion and cook over medium heat until transparent (about 3 minutes).
3. Add the sweet potato, stock powder, water, bay leaf, nutmeg, celery salt and garlic.
4. Bring to the boil, reduce heat and simmer covered for 30 minutes on low heat, stirring every 5-10 minutes.
5. Once the sweet potatoes are soft, remove the bay leaf and purée the soup.
6. Pour back into the saucepan and add the sweetcorn and the evaporated milk.
7. Season with salt and pepper (and cayenne pepper, if desired).
8. Gently heat through, add the herbs and serve in four soup bowls.
9. Serve with roast vegetables, for example roast tomato, onion and zucchini.
This recipe can easily be doubled up if you prefer to use the whole tin of creamed sweet corn and evaporated milk. Chill in the refrigerator and eat within three days, or freeze until needed. The orange-fleshed sweet potatoes give this soup a lovely rich colour.