Method

• 1

  In a large pan heat the onion in the oil, covered but stirring often, until really soft - about 15 minutes. (Cooking the onion really slowly brings out its natural sweetness for the tastiest soup base). Add the tomato puree for 2 minutes to caramelise, then add the chilli, garlic and cumin for 1 minute.

• 2

  Add the sweet potato and stock, bring to a boil, cover and turn down to simmer for 20 minutes, until the potato is tender. Allow to cool slightly before whizzing in a food processor (or with a stick blender) until smooth. For a super-smooth soup, strain through a sieve back into the clean pan. Add the lime juice, season to taste and warm through.

• 3

  Swirl 1 tbsp creme fraiche and herby oil into each bowl of soup then serve.