The original medieval recipe, ‘Rygh In Sawse’ in Forme of Cury (1390) calls for ‘rygh’ which has been interpreted as ruffe, a member of the perch family. I have opted to use trout fillets here although salmon fillets would work too. You could use whole rainbow trout if you don’t mind dealing with the bones (I’m afraid I do). The original recipe suggests serving this dish cold but I prefer it hot.