Prick holes in sausages with a fine skewer or the point of a sharp knife; place in a frying pan just large enough to hold them and cover with water. Cook over medium heat until water evaporates. Remove sausages and brown in a little oil with butter, turning them from time to time until they are cooked through and well coloured on all sides (20 to 30 minutes). In this way the sausages will be well cooked, will remain soft and keep their skins intact. Remove sausages. Then add tomato purée to fat in pan. Cook for a minute or two, stirring; then add
Add cooked and drained white beans, season with salt and pepper and simmer gently for a few minutes to allow beans to absorb flavour. Return sausages to pan and heat through. Serve beans on a serving platter, topped by sausages.