Chef John's Oysters Rockefeller

Ingredients

• 2 cups rock salt or more as needed
• 1 cup chopped watercress
• ½ cup butter, room temperature
• ¼ cup bread crumbs
• 2 tablespoons minced green onion (white and pale green parts only)
• 2 tablespoons chopped celery
• 2 tablespoons chopped fresh tarragon
• 2 tablespoons anise-flavored liqueur (such as Pernod®)
• 1 pinch cayenne pepper or to taste
• salt and freshly ground black pepper to taste
• 36 oysters on the half shell