Spread the rock salt on a baking sheet and arrange the oyster shells on the salt. The salt helps distribute the heat and stabilizes the oysters.
Combine champagne and cream in a saucepan. Reduce over medium heat to about 1 cup, about 10 minutes. Blend the flour into the butter and add mixture a little at a time to the champagne cream while stirring constantly until the mixture thickens. Season with sea salt, pepper and the chopped dill or tarragon.
Remove the sauce from the heat and reserve.
Heat oysters and their nectar in a saute pan 3-5 minutes until the edges of the oysters begin to curl.
Put an oyster in each shell, spoon 2 teaspoons of sauce over each and top with a slice of brie.
Bake oysters 3 to 5 minutes or until the sauce is bubbly and the cheese is melted and browned.
Arrange oysters on the serving plates and garnish with lemon and fresh dill sprigs. Serve with warm crusty bread and remaining Champagne.