Preheat the oven to 400°F.

Rinse the fish inside and out with cold water and pat dry. With a thin sharp knife, make three diagonal cuts about ¼ inch deep and about 1 inch apart on each side of the fish. Season the fish inside and out with salt and the cayenne pepper and fill the cavity with the chopped fennel fronds.

Using a chef’s knife, chop the fennel seeds. Mix the fennel seeds, curry powder, and 1 teaspoon of the softened butter. Set aside.

Place an 18-by-26-inch sheet of heavy-duty foil or parchment paper on a work surface. Rub the remaining ½ teaspoon butter over the foil, leaving a 2-inch border all around. Turn up the edges of the papillote slightly to hold the liquid you will be adding.

Spread half the leek and fennel slices on the foil to make a bed for the fish and dot with about half of the seasoned butter. Set the fish on top and scatter the remaining vegetables over it. Dot with the remaining seasoned butter, and drizzle with the wine.

Seal the papillote by folding the foil in half over the fish. Starting at one folded corner, fold the edges of the foil over in neat pleats; be sure to fold each successive pleat so it slightly overlaps the previous one to make a tight seal. Continue until you have reached the other corner and the edges are tightly sealed. Slide the package onto a baking sheet.

Bake the fish for 25 minutes. Remove from the oven and let sit for 3 minutes.

With scissors, carefully cut the foil along the pleats, taking care to avoid the escaping steam. Fold back the flap to expose the fish. With two large spatulas, transfer the fish to a serving platter. Tilt the papillote so the juices run out of one corner and over the fish.

Spoon some of the cooking juices over each portion and serve.