The earliest records of Brussel sprouts go back only as far as about 1800, and so are generally considered to be out of period. On the other hand, similar cabbage sprouts seem to have been produced in period by other means. They are a biennial plant, and their strong flavour, typical of the brassica family, makes them highly unpopular.
However, they are exceptionally tasty if cooked the right way. I recommend steamed (or in the microwave) with chicken stock and saffron. To season this according to the Forme of Cury, cook with some minced onion, and slit and shredded leek, and then add powder douce. Unlike a lot of other green vegetables they need to be cooked thoroughly so give them a proper blasting.