In a slow cooker, layer the ingredients in the order given; add the water or stock. Do not stir the ingredients.
Cover and cook on high for 4 to 5 hours or on low for 8 to 10 hours until peas are very soft and the ham falls off the bone.
Remove the meat from the bones and chop. Save some of the chopped ham for a garnish, if desired. Remove the bay leaf.
Before serving, mash the peas slightly to thicken the soup or blend the soup in batches for a smoother texture (safety tips below).
Refrigerate or freeze any leftovers.