Pry open the oysters and separate the juice from the meat, reserving both in the fridge. Clean the bottom shells very well and reserve them for later.;
To make the vichyssoise base: Heat the butter and leeks in a small saucepan over low heat and cook until the leeks are softened but not browned. Add the potatoes and water and simmer until the potatoes are tender. Chill and season with salt. Pour into a blender and puree with the cream and blanched spinach to get a light mint green color. Strain through a fine-mesh sieve.
To make the mousse: Whisk the creme fraiche to soft peaks in a mixer. Heat 1/2 cup of the vichyssoise base in a medium saucepan. Add the bloomed gelatin to the base. Fold in the whipped creme fraiche and 1/2 cup more of vichyssoise base.
To make the oyster glaze: Bring 3 tablespoons of the reserved oyster juices to a boil in a small saucepan. Strain through a coffee filter. Add the bloomed gelatin to the liquid. Let cool but not enough to set.
To finish: Fill the 8 reserved oyster shells about 3/4 full with the vichyssoise mousse and set an oyster on top. Keep cool and brush with the still-liquid oyster glaze. Repeat the brushing until the whole oyster has a shine to it. Garnish with a quenelle of caviar, chive blossom, and dill.
This recipe was provided by a chef, restaurant or culinary professional. It has not been tested for home use.