Shuck oysters and reserve oyster liquor.
Wash and finely dice leeks, celery, and radishes. Prepare seasoning, heavy whipping cream, and water.
Melt the butter in a heavy saucepan and add leeks and celery. Warm them up on medium heat for about 2-3 minutes, gently stirring.
Add reserved oyster liquor, heavy cream, water, salt and freshly ground black pepper. Bring to simmering and cook for about 5 minutes on low heat, stirring. The goal is to infuse cream with oyster and vegetable flavors and slightly cook it down. Add some lemon juice and cayenne pepper, adjust to your taste. Add diced radish right before serving. Warm the plates and place oysters in them.
You are in control of the texture of served oysters. If your prefer them raw, just pour hot soup on top of them, garnish with fresh chervil, and serve. I like them to spend some time in hot soup before serving, so they don't reduce the temperature of the soup too fast. You can also simmer them for 2 to 5 minutes to cook them medium to medium-done.