Heat the oil in a pan, then fry the onions and potatoes for 2 minutes. Add the chicken stock and cook for 10 minutes. Add the basil and pesto, then puree the soup.
Whisk the creme fraiche with the curry paste. Pour the soup into four oven-proof soup bowls, add a dollop of the creme fraiche and curry mix, and grill it for 1 to 2 minutes, until the creme fraiche turns a light golden colour.