Cooking en papillote (in parchment) seals in the moisture of fish fillets, yielding a silken entree and perfectly crisp-tender vegetables.
Preheat oven to 450 degrees. Mix lime zest and juice, garlic, ginger, onion, and chiles in a medium bowl. Fold four 20-inch pieces of parchment in half lengthwise. Unfold, and place 1 fillet and 1 head bok choy along each crease. Rub both with 2 tablespoons oil, and season with salt and pepper. Top each fillet with some onion mixture and 2 sprigs cilantro.
Fold parchment over fish, making small overlapping folds along edges and sealing with a paper clip. Place on rimmed baking sheets. Roast until parchment puffs, 10 to 12 minutes. Carefully cut packets, avoiding escaping steam, and serve.