Using an oyster knife, pry open the oyster shells, then remove the oysters. Discard the top shells; scrub and dry the bottom shells. Drain the oysters, reserving the oyster liquor.
In a large saucepan, melt the butter; add spinach, onion, parsley, bread crumbs, Tabasco sauce, Herbsaint, and salt.
Cook, stirring constantly, for 15 min. Remove from heat. Press the spinach mixture through a sieve of food mill; let cool. NOTE: Mixture may be made ahead of time and refrigerated until ready to use.
Preheat oven broiler.
Line an ovenproof plate or platter w/ a layer of rock salt about 1" deep (moisten the salt very slightly). Set oysters in the rock salt, making sure they are level.
Place a little of the reserved oyster liquor on each oyster. Spoon an equal amount of the prepared spinach mixture over each oyster and spread to the rim of the shell.
Broil approximately 5 min. or until the edges of the oysters have curled and the topping is bubbling. NOTE: Watch carefully.
Garnish the plates or platter with the parsley sprigs and lemon wedges.