Method:
Preheat oven to 400°F. Put herbs, oil, celery, carrots, fennel, leeks, salt and pepper into a large bowl and toss well.
Lay a 12- to 14-inch piece of parchment paper on work surface. Fold in half like a book, open and slide one fish fillet inside. As though you're cutting out a Valentine, cut through both paper layers to create a rough half-heart shape around fish, as large as possible. Open the half-heart and top fish with half of the vegetable mixture. Close the "book" and, beginning at the rounded top of the heart, roll up and crinkle the edge toward the fish, working your way around the side of the half heart. When you get to the heart's tip, firmly tuck and twist the end underneath the package. Repeat with second fillet and remaining vegetables.
Bake on a sheet pan until paper is puffy and starting to brown, 18 to 20 minutes. Remove to serving plates and carefully cut open and serve at the table.