Baking parchment, which can be purchased at kitchen-supply stores and some butcher shops, is useful to have on hand. Use any fresh herb you like with the fish.
Heat oven to 350 degrees. Fold a 24-inch sheet of parchment paper in half, and cut out a heart shape about 3 inches larger than fish fillet. Place fillet near the fold, and place a handful of leeks next to it. Drizzle fish with wine and olive oil, and sprinkle with salt, pepper, and chervil.
Brush edges of parchment paper with melted butter, fold paper to enclose fish, and make small overlapping folds to seal the edges, starting at curve of heart. Be sure each fold overlaps the one before it so that there are no gaps. Brush the outside of the package with melted butter. Repeat with rest of fillets.
Put packages on a baking sheet, and bake until paper is puffed and brown, about 10 to 15 minutes.