Soak peas overnight in water to cover or use a quick method. Drain. Melt the butter in a soup pot, add the chopped onion and the ham bone and cook, stirring often, for about 5 minutes. Add the water, peas, and salt, and simmer for about an hour, or until the peas are mushy. Add pepper to taste and correct seasoning. Remove the ham bone, and put the soup through a sieve or food mill. Return to the pot, add the ham scraps, and reheat over low heat, stirring frequently, before serving.