1. Halve the leeks; wash them well and slice thinly; melt the butter in a saucepan, add the leeks and cook gently for 10 minutes;
   peel and dice the potatoes;
2. add the potatoes and the chicken stock to the saucepan containing the leeks and cook for 15 minutes;
3. meanwhile, open the oysters and add their liquor to the saucepan;
4. in a blender, purée all the ingredients from the saucepan.

Presentation

1. place the raw oysters (6 per person), without their shells, into soup bowls;
2. pour the very hot potato-leek mixture over the oysters;
3. top each oyster with a small spoonful of caviar and serve immediately.