The night before you plan to make this soup, soak the feet or hocks in brine (1 tablespoon of salt per quart of water). They should be completely covered. Put the split peas to soak in plain cool water. There should be about 2 inches of water over the peas--they will swell quite a bit. The meat should be put in the refrigeratior overnight; the peas may be, too, but it is less important.
In the morning, take the meat out of the brine, put it in a 8 qt. pot, add the peas, onion and 2 quarts of fresh water. Bring to a boil. Skim off any foam that comes to the top, then reduce the heat and simmer for about 6 or more hours, stirring occasionally so the peas do not stick to the bottom of the pot. The meat should be fallng off the bones, and the soup should be quite thick and fairly smooth.
Take the meat and bones out of the soup and cool slightly. Take the meat off the bones and cut into smaller pieces. Put the meat back into the soup and reheat if necessary. Serve with a small pat of butter in the center of each bowl.