My mother may be 83, but she is still a marvellous cook, and this is one of her specialities, so I hope I do it right. These are her instructions:
Butter, salt, pepper, lemon-juice
Butter the tinfoil and add salt, pepper, and lemon juice. Put the trout down on it, and add more butter, salt, pepper and lemon juice to it. Fold the foil so it encloses the tail section of the trout, but don't close it up so that the steam can escape. Bake for about 20 minutes at 180 C (in a fan oven) - that's 200 C in an ordinary one, mark 6. Open the foil to allow the steam to escape while it cools.
To serve cold, she removes the skin, and sometimes the spine as well ("But it never looks so tidy!") and covers it with sliced cucumber so the joins don't show. With little rolls of smoked salmon on the side, and perhaps some prawns as well (you'll be lucky, in this household - I don't eat prawns!). And lots of different kinds of salads.