Place rock salt, about 1/2-inch thick on a platter and heat in the oven. Place oysters on the half shell on the hot salt. Broil for 5 minutes.
In a food processor, pulse shallots, thyme, spinach and celery. In a saucepan, combine the water, the oyster liquid and the Worcestershire sauce and let boil vigorously for about 5 minutes. Add the vegetables and cook about 20 minutes or until it thickens to a thick sauce. Stir in the butter until melted and remove from heat, stir in the Herbsaint. Pour sauce over each oyster, sprinkle with bread crumbs and return to hot oven for 5 minutes. Serve piping hot on platter on which they are cooked.