Peel and finely dice the onion and ginger. Wash, peel and cut the carrots. Peel off the lime zest with the zest and squeeze out the lime.
Sauté the onion, ginger and lime zest in hot butter. Briefly sauté the carrots, season and sprinkle with sugar. Simmer for another two minutes, then pour in the broth, bring to the boil and simmer for about 20 minutes.
Puree the soup and stir in the crème fraîche. Season to taste with salt, pepper and lime juice.
Peel the avocado, cut in half and remove the stone, dice the pulp, possibly chop the coriander leaves and add to the avocado. Season to taste with salt, pepper and 2 - 3 tablespoons of lime juice.