Halibut en Papillote with Fennel, Olives and Leeks

Position two oven racks near middle of oven. Preheat oven to 425°.

Cut four large sheets of parchment paper (15 to 18 inches long). Fold in half the short way to crease, then open again. For each packet, drizzle a bit of olive oil on one side of the parchment, near the crease. Divide the sliced fennel, leek and olives among the sheets, arranging them in little piles on the oil. Top the vegetables with the halibut fillets, then season with salt and pepper. Top each with two sprigs thyme and two lemon slices. Drizzle 1 tablespoon wine over each fillet and dot each with 1 teaspoon butter.

To close the packets, fold the empty side of the parchment over the ingredients. Then tightly roll up the three open sides, folding and crimping your way around to create a semicircle. The packet should look a bit like a big paper empanada. (A perfect shape isn't the goal, but a tight crimp is.) Transfer packets to two large rimmed baking pans.

Bake until fish is just cooked through and parchment puffs, about 10 minutes, switching positions of pans after 5 minutes. Use a wide spatula to transfer packets to plates. Use scissors to slit tops. Carefully tear open packets to avoid the steam. Garnish fish with a drizzle of additional olive oil and the fennel fronds.