More from de diaetis:
of rapa (beet or turnip)
(…) Therefore they are useful if cooked two times, and both cooking waters discarded, and the third time cooked with very fat meat. (…)
(…) Eaten as a food, it generates twisting and inflation (of the gut) and renders the stomach illaudible, as is attested by its putrid belching, which especially occurs (if eaten) before the meal. Given on an empty stomach, it raises the food and prevents it from descending to the place where it is cooked (in the stomach i.e. digested) by which cause it hardens them to digestion and inhibits the natural virtues of the food from exiting from the interior. That is why it causes vomiting, especially in those who have windiness in the stomach. But if they are taken after the meal, they cause less windiness, especially ascending winds (e.g. burping), but through its heaviness descends below and moves the food to its place of cooking, where it is digested as is proper.
(…) If it is in frequent use, it generates illaudible blood. To temper this, it is good to boil it, and, having discarded the cooking water, the third time it is cooked with lettuce, coriander, and onions, and seasoned with oil of unripe olives, vinegar, pepper, and caraway. (…)
p. 466 ff
Again, some useful cooking advice and potentially, recipes. Identifying the root vegetables that are supposed to be rapa or napus is always tricky since the family is very broad.
Isaac Iudaeus de diaetis universalibus et particularibus, originally written in Arabic in the late ninth or early tenth century, was translated and adapted by Constantinus Africanus in the late 11th century and circulated widely in Italy and beyond soon afterwards. While the original applies to a different context, it is still reasonable to use it as a guide to the advice that Siculo-Normans would have found useful. It is an open question how much the original was altered in translation – I cannot say since I read no Arabic. However, the extensive reference to eating pork suggests that some alterations took place.