Cut a circle of greaseproof paper or baking parchment approximately 25cm wide for each piece of fish. Fold each piece in half and tuck the fillet in next to the crease.
Sprinkle a little salt and pepper on and add the knob of butter, if using.
Starting at one side of folded over paper, make small folds all the way around to create a parcel and seal it up – it will look a bit like a Cornish pasty.
Put on a baking tray and cook for 8-10 mins if the fish is 1-2cm thick, 12-15 mins if it is 3-4cm thick.
It is perfectly acceptable to serve these fish rare; in fact if they are overcooked they become quite chewy, so again, less is more.
Recipe by appetite magazine at https://www.appetitemag.co.uk/wild-salmon-and-sea-trout-en-papillote/