Heat the oil in a large pan. Add the spring onions, garlic, and gently cook for 5-7 minutes or until the spring onions are soft but not brown. Pour in the stock and bring to the boil. Add the litchis, bring back to the boil and reduce to simmering point. Stir in the lettuce and mint.
Continue to cook for 2-3 minutes or until the litchis are just tender. Allow the soup to cool and then blend in a food processor or with a hand blender until smooth. Season to taste with salt and pepper. Pour into a large bowl, add cream, cover and chill for four hours or overnight.
To serve, spoon the soup into chilled soup bowls. Spoon a tablespoon of cream into the soup and top with a sprig of mint before serving.