1. Preheat oven to 325°. Blend marinade ingredients until very smooth. Pour into a bowl, add fish, and gently turn to coat. Rinse blender and dry.
2. Arrange fillets on a rimmed baking sheet and bake until they flake slightly when gently pressed, the last bit of translucence is fading from their centers, and a bit of liquid collects around them, 12 to 14 minutes.
3. Meanwhile, make sauce: Put wine, shallots, and lemon juice in a small saucepan and bring to a boil over high heat. Cook until reduced by half, about 4 minutes, then whisk in butter, 1 tbsp. at a time. Pour into a blender and whirl briefly, then add 3/4 cup chervil and pulse until finely chopped.
4. Pour sauce into 6 shallow bowls. Top with fish, add salt to taste, and garnish with chervil and flowers.