Cooking fish in an aluminum-foil packet is a sensible option for several reasons. Cooked fish flesh is often delicate, and a foil wrapper keeps the fish safe from overhandling. Foil holds in juices, marinades and flavors, preventing fish from drying out. Foil also makes for easy cleanup once the meal is over.
Measure the thickness of the fish. Figure on 10 minutes of cooking for each inch of fish thickness. Heat the oven to 425 F.
Spread a sheet of aluminum foil over a baking pan and spray it with non-stick cooking spray.
Prepare any marinade or flavored cooking liquid you prefer. When you cook fish in aluminum foil in an oven, you are essentially steaming the fish, not baking or roasting it. This means you need to add some melted butter, marinade or cooking juice to the foil packet.
Cut out one piece of aluminum foil for each piece of fish you plan to cook. Make each piece about 1 inch longer than the longest piece of fish and at least twice as wide as the widest piece.
Lay each fish piece on its own sheet of aluminum foil, along with at least 1 tbsp. of cooking juice and, if desired, cut vegetables. Wrap the package tightly and place it on the foil-covered baking pan.
Bake for the prescribed amount of time, according to fillet thickness. The fish is ready when it is opaque and flaky. It should fall apart easily when you probe it with a fork.