I love the idea of someone in medieval costume, sitting beside a bubbling brook. Beside them rest a book, and half a bottle of wine. A fishing rod made out of a stick and string held loosely in hands, while they napped under a large hat.
And accidentally catching a trout and ruining the quiet of the day.
Took cook your trout you need to scrape the scales on the outside of the fish with the side of a knife and wash them off the fish, the knife, your hands and everything you touched in the last 24 hours. You then cut the fish’s belly open from anus to neck and remove all the squishy guts from the pocket there, and wash the fish again. Pat dry.
You can grill a 1/2 lb trout on a high (500F) heat and it will cook in about 5 minutes. Pan frying is about that time, on each side.
Boiling takes a little longer, 10 minutes for every inch thick your fish is. The medieval cook would put 2 cups of wine or vinegar into the water with the fish.
In all cases cooked fish flake away from the bone easily.
The medieval cook will pair the fish with a sour sauce of your choosing:
Le Viandier de Taillevent (1380):
Salmon trout. Peeled, head and all, and then cooked in water or roasted; eaten with verjuice.
Libre del Coch (1520)
And if they want to eat it roasted, it must be eaten with orange juice and water and salt and a little oil, and all the good herbs; and prepare the plate of the roast trout. And cast this juice on top of everything. And know that the best morsel of it is the snout.