Check the fish for any stray bones and cut off any dark flesh that was next to the skin. Cut the fillet into 4 equal square pieces. Season both sides of the fish with salt and pepper. Separate the outer leaves from the head of lettuce, without tearing them. Place a leaf cupped side up and center a piece of fish in the middle of the leaf (use two leaves if necessary). Sprinkle the fish with 1 tablespoon of the dill. Top with another 1 or 2 lettuce leaves, this time cupped side down, so that the fish is completely enclosed in lettuce. Wrap the remaining fillets in lettuce, sprinkle each one with dill, and arrange them, without crowding, in a steamer rack or basket. Bring several inches of water to a boil in the bottom of a steamer. Drop in the mint sprigs, put the steamer rack or basket in place, cover and steam fish 8 minutes for each inch of thickness (e.g. if fish is 3/4 inch thick, steam 6 minutes). Turn off the heat, uncover the steamer and let the fish rest for 1 to 2 minutes. To check for doneness, carefully lift the top leave from one of the packets and peek inside the fillet. If it is still translucent in the center, cover and steam for another 1 to 2 minutes.
While the fish is steaming, make the sauce. Melt 1 tablespoon of the butter in a small saucepan or skillet over medium-low heat. Add the shallots and cook, stirring, until it is softened but not browned, about 1 minute. Add the lemon zest and juice and bring the mixture to a simmer. Vigorously whisk in the remaining butter, 1 tablespoon at a time, allowing each addition to melt before adding the next. The sauce should be slightly thickened and the butter emulsified. Stir in the dill and season with the salt and pepper to taste.
Carefully transfer the fish packets to warm serving plates. Pour the sauce over and around the fish and garnish with the dill flowers.