Step 1Fill the bottom of a steamer with 1 to 2 inches of water. Set aside.
Step 2Combine the vinegar, wine and shallots in a 3-quart saucepan. Heat the mixture to simmering and cook over medium heat until the liquid is reduced to 1 tablespoon.
Step 3Remove the saucepan from the heat and whisk in 1 piece of the butter.
Step 4Return the pan to low heat and continue beating in one piece of butter at a time. When you have added about half of it, heat the steamer.
Step 5Continue to add the butter and beat the sauce in the pan over low heat until all the butter has been incorporated and the sauce is creamy and light.
Step 6Remove the sauce from the heat and stir in salt, pepper and 1 tablespoon of the chopped lemon verbena leaves. Cover and set the sauce aside while steaming the fish.
Step 7Put 2 fillets in the steamer. Top each with a sprig of verbena. Cover and steam until the fish is opaque, moist and tender, 3 to 4 minutes. Repeat with the remaining fillets.
Step 8Carefully remove the fish to a serving plate. Sprinkle with fleur de sel to taste and spoon the verbena beurre blanc over. Sprinkle with chopped verbena.