Peel the potatoes and cut into cubes. Halve the leek lengthways and cut into fine strips.
Heat the butter in a pan, sauté the potatoes, leek and grated lime zest in it, add the chicken stock, bring to the boil, cook over a low heat for about 30 minutes.
Puree the soup. Stir in 2/3 of the crème fraiche, allow to cool, then chill for at least 2 hours. Season to taste with salt, pepper and a little lime juice.
Arrange the soup in ice-cold bowls or soup plates, garnish with the remaining crème fraiche and salmon caviar.