Here's the original recipe from 1390 with my modern reinterpretation below.
Take Capouns and seeþ hem, þenne take hem up. take Almandes blaunched. grynd hem and alay hem up with the same broth. cast the mylk in a pot. waisshe rys and do þerto and lat it seeþ. þanne take brawn of Capouns teere it small and do þerto. take white grece sugur and salt and cast þerinne. lat it seeþ. þenne messe it forth and florissh it with aneys in confyt rede oþer whyt. and with Almaundes fryed in oyle. and serue it forth.
In modern English:
Take capons and seethe (boil) them, then take them up. Take blanched almonds. Grind them and chop them up with the same broth. Put milk in a pot. Wash rice and add thereto and let it seethe (boil). Then take the flesh of the capon, chop it small and add thereto. Take white grease (lard), sugar and salt and put them in. Let it seethe. Then mix it up and garnish it with any sweetmeat red or white, and with almonds fried in oil. And serve it forth.
Many different mediæval versions of this recipe exist, often incorporating ginger and saffron. Blanc Mang eventually evolved into blancmange, which was a staple of school dinners when I was a child.
Blanc Mang - my modern version
Stir-fry chopped onions and garlic, add chopped chicken breast and nutmeg and cook through. Cover and take off the heat.
Cook rice with chopped ginger, stock and ground almonds. Once all the liquid has been taken up and the rice cooked, combine with the cooked chicken. Top with chopped coriander, fried flaked almonds and crystallised ginger.
Dine like Richard II and enjoy!