Bring a large saucepan of water to a boil. Add chayote, onion and leek and boil until the chayote is tender, for about 10 minutes. Remove from heat and allow it to cool. In a food processor puree the chayote mixture and add chicken stock, cream, fennel seed, and season, to taste with salt and pepper. Set aside until later.
In a food processor puree until smooth the cilantro with 1/2 cup of water. In a bowl mix the cilantro paste with the crema fresca thoroughly and season to taste with salt and pepper.
Place chayote soup in a serving bowls and place a dollop of crema fresca on top.