How to Make It
Melt the butter in a large saucepan. Add the leeks and cook over low heat until tender, about 8 minutes. Add the cauliflower and water and bring to a boil. Simmer over low heat until the cauliflower is soft, about 35 minutes. Add the crème fraîche, bring back to a simmer and remove from the heat.
Puree the soup in a blender in batches until very smooth. Season with salt and pepper and chill thoroughly.
Just before serving, combine the cucumber, chives, shallot, vinegar and olive oil in a small bowl. Ladle the cold soup into 4 shallow bowls and spoon an oyster and some of its liquor into the center of each. Spoon the cucumber mixture around the oysters, top with a dollop of osetra caviar and serve.