The hardest thing about the recipe is tracking down Verjuice. Verjuice is unfermented wine grape juice and is a common ingredient used in medieval and renaissance cooking. It has come back into modern cookery quite recently and adds a very special taste to dishes. In Australia, Maggie Beer produces verjuice commercially - sadly I have yet to find a supplier here in Europe.
At a total pinch you could use a very mild vinegar - I had a bottle of verjuice that a friend sent me and with experimentation, I made up a mixture of grape juice and wine vinegar and was able to get something that resembled verjuice for the feast, as my little bottle certainly didn't contain enough to feed 138 people!
Somewhat unusually for an ancient 'receipt', this recipe does have some guidance as to quantities of ingredients. My version used this as a guide but I made it with duck meat rather than a whole bird.
To bake a Mallard (The Good Housewife's Jewell 1596)
Take three or foure Onyons, and stampe them in a morter, then straine them with a saucer full of vergice, then take your mallard and put him into the iuyce of the sayde onyons, and season him with pepper, and salte, cloves and mace, then put your Mallard into the coffin with the saide juyce of the onyons, and a good quantity of Winter-savorye, a little tyme, and perselye chopped small, and sweete Butter, so close it up and bake it.
Bake in a closed pie shell, or as little individual pies. Eat while piping hot.