Habas (o guisantes) frescas 
con caldo de carne

Note: Instructions in normal letters are pretty clear in the original. My interpretations are in italics.

Take:

• fresh broad beans or peas or any other (period European) fresh legume, taken out of their sheaths (optional)
• salted meat or a reasonable substitute, or broth
• a bunch of fresh parsley (of the broad-leaved southern european tasty variety – not a garnish!), finely chopped
• a bunch of mint (I think this refers to sweeter spearmint rather than peppermint but I could be wrong), finely chopped

Cook the beans or peas or whatever either in broth or with some salted meat. When they are done add the parsley and the mint. The soup should be somewhat green to be pretty.
 

My experiences

I later ask about suitable substitutes for the salted meat at the Rialto (rec.org.sca) and got the following options:

• use Beardsley sliced Dried Beef ("like the Hormel stuff but much better"), available in glass containers, apparently used to make Creamed Chipped Beef on Toast and thus available all over the US – Can’t comment on this for geographical reasons. 
• get some thin-sliced plain roast beef, or similar. Then paint an oven rack with oil, spread one layer thick, and dry on "warm", checking frequently. Or just use a dehydrator.

I will be checking recipes for salted meat in an effort to come closer to the real thing. If you have any ideas let me know.

Links on preserving meat by salting:
The Roman way: http://www.unc.edu/courses/rometech/public/content/survival/Lindsay_Mack/Food_Preservation.htm
The modern, almost-the-same, way:
http://waltonfeed.com/old/brine.html
 

Update

It seems that medieval soups were generally thick affairs, not as liquid as we take them now. I got this from either Scully's "Art of Cookery in the Middle Ages" or Barbara Santich's "The Original Mediterranean Cuisine

The result, for the time being

Since I wrote this I have also tried it with fresh green peas and with fresh broad beans. both were delicious!