Melt the butter in a large saucepan over low heat.

Add the thinly sliced leeks to the saucepan.

Cook the leeks until they become tender, approximately 8 minutes, stirring occasionally.

Add the cauliflower florets and water to the saucepan.

Bring the mixture to a boil.

Reduce the heat to low and simmer until the cauliflower is soft, about 35 minutes.

Stir in the creme fraiche.

Bring the mixture back to a simmer, then remove from the heat.

Carefully transfer the soup in batches to a blender.

Puree the soup until it is very smooth.

Season the pureed soup with salt and pepper to taste.

Chill the soup thoroughly in the refrigerator for at least an hour.

In a small bowl, combine the diced cucumber, minced chives, minced shallot, rice vinegar, and olive oil.

Stir the ingredients together gently.

Ladle the cold soup into four shallow bowls.

Place a shucked oyster and some of its liquor into the center of each bowl.

Spoon the cucumber mixture around the oysters in each bowl.

Top each serving with a dollop of osetra caviar, if desired.

Serve immediately.