Mix crabmeat, celery, chervil and lemon zest in a bowl. Cover and chill crab salad.
Halve and pit avocados; scoop flesh into a blender.
Add vegetable stock, crème fraîche, lime juice, kosher salt, and 1 1/4 cups water. Purée until smooth. Season soup to taste with salt and pepper; chill.
Divide soup among 4 bowls. Spoon crab salad into center of each bowl.