“And first a SOUP of OLD PEAS. It is appropriate to shell them, and to find out from the people the place the nature of the peas of the area (for commonly peas do not cook well in well-water: and in other places they cook well in spring-water and in river water, as in Paris, and in other places, they do not cook at all in spring-water, as at Besiers) and this known, it is appropriate to wash them in a pan with warm water, then put in a pot with warm water on the fire, and boil them until they burst. Then separate the liquid from the solid, and put the liquid aside, then fill the pea-pot with warm water and put on the fire and separate a second time, if you wish to have more liquid: and then put back without water, for they will produce enough. and boil in it; and it is not appropriate to put the spoon in the pot after the separating, but shake the pot and the peas together, and little by little feed them with warm water or a little more than warm but no cold, and boil and cook completely before you add anything except hot water, be it meat or anything else: do not add salt, nor bacon, nor absolutely anything whatsoever until they are fully cooked. You can add bacon water or meat stock, but you must not add any salt, nor even the tip of the spoon, until they are well cooked; you can always stir them by moving the whole pot.
On meat days, you should, after the separating, add water from bacon and from meat, and when it is almost cooked, you can put bacon in; and when you remove the bacon from these peas, you must wash it with meat-stock, so that it looks nicer to put in slices on the meat and so that it does not appear to have peas stuck to it.”
Source: Le Menagier de Paris
I wonder what the modern explanation for the peas not cooking well in various types of water is.
This sounds like a pretty easy recipe to try, and pretty much like modern split pea soup, at least the way I cook it.