Ribollita (lit. "reheated", "boiled again") is a popular Tuscan dish, a thick soup of bread and vegetables.
As far as I know, we do not have any written sources that attest it in the Middle Ages -- in fact, the oldest recipe I know is from the 1930s. However, the local legend would have you believe it was made by servants, who used leftover bread from their masters' tables to thicken their vegetable soup.
While this may well be just a legend, we do have plenty of evidence that lower-class Europeans made all sorts of thick broths with grains, flour, and possibly stale bread. While we have no direct evidence for this particular dish, it's not at all unlikely that some people in the Middle Ages would have eaten something like this.
Don't sweat it about the vegetables, use whatever you have around. Modern recipes use zucchini and canneloni beans but those weren't available to Europeans in the Middle Ages.
Dice the carrots, turnips, leek and celery. Wash and dice the mushrooms. If you're using cabage, shred it. Slice the onion halfway through. Boil all these together in as little water as possible until you get a good, flavourfoul soup.
Take it off the stove when all vegetables are soft. You can take out the onion now if you don't like it, but I usually just chop it a little and throw it back in.
Add the sliced bread, one or two slices at a time, and mash it along with the vegetables. You can use a potato masher. Some of the vegetables are inevitably going to get mashed, too -- don't worry about it, it's part of the fun. You don't have to completely mash the vegetables, this isn't supposed to be a cream-soup. Once the soup is thick -- it should be thicker than a soup, but not as thick as mashed potatoes -- throw in the black-eyed peas and the kale, if you have it, put it back on the stove and stir well. Leave it for another 5-10 minutes to steam the kale and the peas a bit, adding some more water if necessary to keep it from burning.
Serve hot with olive oil. Modern serving etiquette calls for grated Parmigiano or Pecorino. Tuscan peasants in the Middle Ages probably didn't do that, and I'm not sure what was worse for them -- the fact that antibiotics were unknown or the fact that they couldn't grate cheese over ribollita. Seriously, grate some cheese on top of it.