While on my knees lately harvesting onions, I thought to pass on this little gem:
Togli cipolle; cuocile sotto la bragia, e poi le manda, e tagliale per traverso longhette et sottili: mettili alquanto d’aceto, sale, oglio e spezie, e dà a mangiare.
TRANSLATION: Take onions; cook them in the embers, then peel them and cut them across in longish, thin slices; add a little vinegar, salt, oil, and spices, and serve.
REALIZATION: This is the simplest of simple dishes, and drives home how important the quality of ingredients is to any dish. Buy as many sweet onions as you choose for the meal. If the onions have a fairly thick, firm, papery skin, you can just bake them uncovered in z shallow pan in a 450º oven for about an hour, checking frequently during the last 15 minutes or so, to prevent the skins from catching fire as they bake. If the onions are the white sweet onions that are in season locally about this time, they have virtually no papery skin, and benefit from wrapping in a single layer of foil. Twist the foil at the top of the onions rather than wrapping as a package, so that there is as little thickness of foil as possible over the body of the onion. When baked, remove from the oven and cool until you can handle them – they will be very soft.. Peel only enough layers to take off the dried skins. If some of the layers are caramelized brownish, bur NOT DRY, leave them on, because they will be the sweetest. Slice the onions cross-wise into thin slices, and put into a serving bowl. Add salt and olive oil to taste, spices if desired, and vinegar to taste. Serve to appreciative onion aficionados.
The recipe itself offers no hint on the spices, so you may season to taste. However, Apicius often serves ginger with onions. Another Tuscan recipe book of the same vintage (also used in The Medieval Kitchen) offers a spice mixture of 8 parts each of pepper, cinnamon and ginger, one part cloves, and two parts saffron, which will give a bit sharper flavor than plain ginger.
The authors of The Medieval Kitchen note that this recipe “appears at the end of our Tuscan collection, after the recipes for invalids.”  Looking at the provenance for some hints on ingredients, I note that Tuscany is the area just south of Emilia-Romagna, the two states in north-central Italy, nestled between the arms of the Arno and Po rivers. Emilia-Romagna is the state containing Modena, traditional home of balsamic vinegar. Could this have been the vinegar used in the Tuscan collection? Speculative, but very possible.
Balsamic vinegar is made from boiled must (see glossary), which is then evaporated in the process of transfer through wooden kegs of decreasing size over many years. When finished, balsamic vinegar can be almost the thickness of honey, In about the year 1100, a sweet product called “balsamo” was used medicinally, and from the 12th to 14th centuries, there were guilds to guard the secret of its manufacture.  Apicius also used grape juice concentrated by boiling, often to half or less of the original volume, to sweeten dishes. He called this concentrated wine “sapa”.
In the Oxford English Dictionary, 2nd ed., the word “balsamic” is not really documentable in period, but is in wider use by the mid 16th century, and is in common use by the 19th. However, the word “balsam” is used to describe a sweet aromatic substance, frequently used as medicine, going all the way back to Old English. I can testify from personal experience that balsamic vinegar is superb with this dish. But if you use good balsamic vinegar, go easy on the spices, or leave them out entirely. Good balsamic vinegar suffers no competition for flavor.
Must: The expressed juice of fruit, and especially grapes, before and during fermentation; also : the pulp and skins of the crushed grapes.
 Odile Redon, et al., The Medieval Kitchen, p. 79-80
 Pamela Sheldon Johns, Balsamico!, Ten Speed Press, Berkeley, 1999, p. 22.