Heat the butter in a heavy-bottomed skillet over medium-high heat. When the butter begins to bubble, add the celery, fennel, shallot, garlic, parsley, and watercress. Cook this mixture for 5 minutes, stir often to prevent scorching. Remove the pan from the heat, and carefully add the Pernod, then return the pan to the heat (the Pernod may flame). Simmer for 3-4 minutes, then stir in the cream, salt, and pepper. Bring the cream to a boil, and simmer it until it has reduced by half, and has begun to thicken the sauce. Remove the pan from the heat and stir in the breadcrumbs; allow the Rockefeller topping to rest for 5 minutes. Transfer the topping to a food processor and puree. Preheat an oven to 450 degrees Fahrenheit.
Spread the coarse salt onto a baking sheet that is large enough to hold the oysters. Rest the oysters on the salt, making sure they are stable. Divide the topping evenly amongst the oysters, making sure to coat the entire surface of each oyster. Bake the oysters for 7-10 minutes, or until the surface of the topping is bubbling and a desired doneness is achieved.