Preheat oven to 450F. Fill 4 pie or cake tins (or a baking dish large enough to hold oysters) with rock salt, but no more than half full (use just enough salt to keep the oysters from rocking back and forth).
Saute the shallots, celery, and parsley in 4 tablespoons of butter in a heavy skillet for approximately 57 minutes.
Add spinach to the skillet and allow it to wilt for a minute.
Pour spinach mixture into a blender.
Add the remaining butter, bread crumbs, Worcestershire sauce, salt, peppers, and Pernod or Anisette. Blend for a minute at medium speed. Top each oyster with about 1 teaspoon to 1 tablespoon of the mixture, depending on the size of the oyster.
Remove the tins from the oven and embed the oysters firmly in the hot salt. Return pans to the oven and bake for about 4 minutes, or until the butter is melted and the spinach is lightly browned on top.