Cooking fish in a sealed, airtight pouch is one of the easiest and healthiest preparation methods. Referred to as cooking "en papillote" -- the French term for the technique -- it usually calls for wrapping the fish in a parchment parcel and baking it in the oven. Foil works just as well as parchment, with the additional benefit that it's suitable for the grill. You can also use this method to cook fish straight from the freezer, putting a tasty meal on the table in just minutes.
Rinse the frozen fillet portions under cold running water to remove any surface frost or icy glaze from the fish. Pat them dry with clean paper towels.
Tear a sheet of aluminum foil, making a rough 12-inch square for each portion. Brush or spray the foil lightly with your preferred cooking oil.
Arrange each fillet in the middle of a sheet of foil, then lift up the edges of the foil and crease them together to make a seal. Do the same for each end, creating a loose package for each fillet.
Preheat your oven or grill to 425 degrees Fahrenheit. Place the frozen fillets in their pouches on the grill's grate, or arrange them on a baking sheet and slide them into the oven.
Cook the fillets for approximately 8 to 10 minutes, depending upon the thickness of your fillets and the number of fillets you're cooking. Remove the fish from your oven or grill and open the packages, then season the thawed fillets to your taste. Re-seal the packages and cook for another 8 to 10 minutes.
Remove your fish from the oven or grill, and open the packages again. Carefully transfer each fillet to a plate. The juices in the package can be used to make a sauce or simply dribbled over each fillet.