Heat the butter in a sauté pan. Add the parsley, celery, fennel, shallots and garlic and cook for 5 minutes.
Add the watercress and cook for 1 minute.
Add the bread crumbs; season with salt and pepper.
Transfer the mixer to a food processor and puree.
Top each oyster with approximately 2 tsp of the mixture; it should cover the surface of the oyster.
Bake the oysters on a bed of rock salt at 450* until the mixture bubbles, approximately 6-7 minutes.