Cooking with foil packets is a sort of compromise between baking and steaming. Moreover, it is a quite easy and quick cooking method. Another advantage? You can cook with foil packets (almost) any food. Besides that, it also allow you to cook with only a little grease and without alterating food taste and nourishing facts!
In order to make a foil packet, you can use aliminum foil, or baking paper, or even som types of vegetal leaves, like those from fig tree, or babana tree, or vine. Or else, you can also use a special type of silicone containers.

You can use different techniques according to the food are about to cook.

If you want to cook a whole fish.
Cut a rectangular sheet of baking paper, put the fish in the middle, fold the longest sides over the food (overlapping them), fold the remaining sides as well, and secure the packet with some kitchen twine.

If you want to cook fillets, fish steaks, vegetables, pasta, etc.
Cut a sheet of baking paper, place the food in the middle, lift the edges upwards, close the packet by tying it with kitchen twine (like a sort of envelope for an Easter egg).

If you want to cook big portions.
In this case, you may want to cut two square sheets of baking paper, and overlap them. Put the food in the middle, join the edges and fold them on the four sides, even more than once, so that the packet will be securely  closed.

Whatever techniques you choose to use, pre-heat your oven at about 200°C and add some liquid seasoning (stock, wine, sauce…) inside the packet.