This soup is made with dried peas; these generally have to be soaked overnight, though it is possible to buy some that need only a few hours' immersion. It is slightly less trouble to make the soup with split peas - which have no skins - and here there is a choice of green or yellow. Although there is no difference in the taste, the latter give the soup a pleasant golden colour.
Soak the peas as directed on the packet. Chop the onion, if used, and soften in a little fat over a low heat. Add the peas and water or stock and the ham bone if used.
Cook gently until the peas are soft - about an hour. Remove the bone and strip off any meat. This should be cut into small dice and reserved. Puree the peas in a blender or pass through a sieve. Adjust the seasoning. Add the diced ham and serve with a swirl of cream or a sprinkling of chopped parsley on top.