Preheat the broiler. In a mixer bowl, using an electric mixer on medium-high speed, beat the creme fraiche or sour cream until soft peaks form. Stir in the tarragon, lemon zest, orange juice concentrate, salt, and cayenne; set aside.
In a jelly-roll pan, layer the kosher or rock salt 1/4-inch thick. Gently press the oyster shells into the salt to hold them in place. Broil the oysters 6 inches from the heat source for 4 to 5 minutes, or just until they begin to release their juices and the shells pop open slightly. Remove from the broiler and remove the top oyster shell by inserting a butter knife and popping the shell off. Top each oyster with a spoonful of creme fraiche topping and serve immediately.
Recipe courtesy of Cory Schreiber, Wildwood, Ten Speed Press, 2000