Wash and soak peas in water to cover, overnight. In the morning drain off the water and cover with 3 litre of fresh water. Add the ham bone and onion and cook until peas are soft. Rub through a sieve. Melt the butter and stir in the flour until well blended and smooth. Add the salt, pepper and milk and cook, stirring constantly until the mixture thickens. Combine with the strained liquid and cook until rather thick. Serve hot.