Cooking fish en papillote, or folded in a pouch, is a classic French technique that, in addition to being incredibly easy (and virtually cleanup-free), allows the fish to steam in its own juices and thus emerge moist and flavorful. We found that foil was easier to work with than parchment and created a leakproof seal. Placing the packets on the lower-middle rack of the oven, close to the heat source, concentrated the exuded liquid in the packets and deepened its flavor.
Vegetable selection was important: Hardier vegetables like potatoes and squash failed to cook evenly in the packets, and more water-absorbing vegetables like eggplant turned to mush when enclosed. Carrots and leeks, cut into elegant matchsticks, cooked at the same rate as the fish and made a nice presentation as a bed for the fish when everything emerged from the packets. A zesty compound butter topping the fish added richness and flavor (and color contrast to the fish) as it melted.
Technique tip: Open each packet promptly after baking to prevent overcooking. To test for doneness without opening the foil packets, use a permanent marker to mark an "X" on the outside of the foil where the fish fillet is the thickest, then insert an instant-read thermometer through the "X" into the fish to measure its internal temperature.
Swap options: Try black sea bass, haddock, hake or pollock.
Adjust oven rack to lower-middle position and heat oven to 450 F. Mash butter, thyme, half of garlic, 1/4 teaspoon lemon zest, 1/4 teaspoon salt and 1/8 teaspoon pepper in bowl. Combine parsley, remaining garlic, and remaining 1 teaspoon lemon zest in second bowl. Combine carrots, leeks, 1/4 teaspoon salt and 1/8 teaspoon pepper in third bowl.2.
Lay four 16- by 12-inch rectangles of aluminum foil on counter with short sides parallel to counter edge. Divide vegetable mixture evenly among foil rectangles, arranging in center of lower half of each sheet of foil. Mound vegetables slightly and sprinkle with wine. Pat cod dry with paper towels, sprinkle with remaining 1/2 teaspoon salt and remaining ¼ teaspoon pepper, and place on top of vegetables. Spread butter mixture over fillets. Fold top half of foil over fish, then tightly crimp edges into rough 9- by 6-inch packets.3.
Place packets on rimmed baking sheet (they may overlap slightly) and bake until cod registers 135 F, about 15 minutes. Carefully open packets, allowing steam to escape away from you. Using thin metal spatula, gently slide cod and vegetables, and any accumulated juices, onto individual plates. Sprinkle with parsley mixture and serve with lemon wedges.