The book was compiled by master cooks who worked for Richard II
Chefs searching for an authentic medieval way to cook a porpoise can now look up the recipe online.
The Forme of Cury, compiled by master cooks to Richard II, is part of a collection of medieval texts held by the John Rylands Library, Manchester.
Now an edition of the cookbook dating from the early 15th Century, compiled in about 1420, has been digitalised and uploaded to the library's website.
John Hodgson, keeper of manuscripts, said it contained hundreds of recipes.
Among them are exotic dishes featuring porpoise and more recognisable names like blancmange.
Mr Hodgson said the latter was different to the modern interpretation - a rice dish, highly spiced and sugared.
Such ingredients were extremely expensive and beyond the income of most ordinary people during Richard II's reign.
The recipe begins "For to make blanc mange" and goes on to say "put rice in water all night and in the morrow, wash it clean".
"It's not a like a modern cookery book so it doesn't give you exact quantities and times," said Mr Hodgson.
"It's very much suck it and see, but great for experimenting.
"The complete book - all 100 pages - is now available online so that anybody who is interested in cookery, well, you could actually make some of the recipes now."
The original manuscripts which make up the Forme of Cury are thought to date back to 1390.