1 tablespoon extra-virgin olive oil
3 cups thinly sliced leeks
1 garlic clove, finely chopped
3 cups vegetable or chicken stock
1 large ripe avocado
1/2 cup cultured sour cream
1 tablespoon fresh lime juice
2 tablespoons chopped fresh cilantro
Salt and fresh-ground pepper
Heat the oil in a saucepan. Add the leeks and garlic and cook, stirring frequently, until the leeks are soft but not browned, about 10 minutes. Pour in the stock and bring to a boil. Cover the pan, reduce the heat, and simmer until the leeks are tender, about 10 minutes.
Remove the soup from the heat and leave to cool slightly, then puree it in a blender or food processor. (Alternatively, the soup can be pureed in the saucepan with a hand-held blender.) Pour the soup into a bowl and leave it to cool. Cover and chill well.
Just before serving the soup, prepare the avocado. Halve the avocado and discard the seed. Scoop the flesh from the peel and press through a fine stainless-steel or nylon sieve. The avocado can also be pureed in a blender or food processor until smooth, adding a little of the chilled soup to thin the puree and make sure it is smooth.
Stir the avocado puree into the soup together with the sour cream, lime juice, and cilantro. Add seasoning to taste, then ladle the soup into 4 bowls. Add slices of lime and cilantro sprigs.