Soak and simmer the peas: Place the peas in a pot with the water and bring to a slow simmer. Cook until soft and beginning to break down, about 30–40 minutes. In medieval kitchens, this would have simmered for hours over a hearth.
Add vegetables: Stir in the onion, garlic, and root vegetables. Continue simmering until the mixture thickens and the peas begin to dissolve, about 20–30 minutes.
Add greens and thickener: Add the chopped kale or cabbage. Stir in the bread or grains to thicken. Continue simmering until everything melds together into a hearty, porridge-like consistency.
Finish: Season lightly with salt if available. For a more authentic finish, stir in a small spoonful of rendered fat or a splash of ale for depth.
Serve: Spoon into a wooden or earthenware bowl. Serve with coarse barley or rye bread, a wedge of hard cheese, and a mug of small ale or cider.