Place the chicken in sufficient simmering water, cover, and simmer very slowly for 2 1/2 hours. Skim during the first 15 minutes. Cool the chicken, dismember, and arrange the pieces on the serving dish.
Finely chop the ginger. Grind the ginger, grains of paradise, cloves, and saffron in a mortar. Set aside.
Soak the cassia flowers in several changes of cold water to remove the excess salt taste.
To make the sauce, finely chop the cassia flowers, sage, parsley, and bread. Crush in a large mortar (see Note 3), adding the vinegar as you go.
Add the ground ginger, grains of paradise, cloves, and saffron to the sauce. Crush and mix thoroughly, and strain through cheesecloth (see Note 4).
Pour the sauce over the chicken on the serving dish. Garnish with thin slices of hard-boiled egg yolk (see Note 5).
Note 1: The original recipe presumably referred to dried cassia flowers (buds). I found the cassia flowers in brine in a Chinese supermarket. The tin contained mostly cassia leaves, so I carefully picked out all of the cassia flowers, and then added smaller cassia leaves until I had the full measurement.
Note 2: Modern tastes may not enjoy this recipe if full strength vinegar is used. If so, take 1/2 US cup (115 ml) of white vinegar and cut it with 1/4 US cup (60 ml) of water or chicken stock.
Note 3: If you don't have a large mortar then use a blender or food processor.
Note 4: If you happen as I do to like sauce with texture, do not strain it, but do inspect it carefully to remove large lumps.
Note 5: The original recipe is confused about what is to be done with the egg yolks. I interpret them as a garnish. If after garnishing you feel that the dish needs one final touch, sprinkle it with some paprika (though this was unknown in Europe in 1390).
The original recipe (in translation) is here