Preheat the oven to 400 degrees F.
Season the fillets well with salt and pepper. Using parchment paper, fashion 4 packages for two fillets each. Place 2 fillets in the center of each piece of parchment paper. Drizzle with some of the olive oil and divide the onion, garlic, pepper, and fennel fronds evenly among the packages. Sprinkle each fillet with some of the wine, seal the packages by folding the paper over, and, if necessary, sealing the edges with beaten egg whites, place each on a baking tray and cook in the oven for 10 minutes. Remove the packages, slash with a knife to let the steam escape and serve the fish and its cooking liquids immediately.
Recipe copyright 2001, Mario Batali. All Rights Reserved.