1. Peel the onions carefully, trimming only the minimum at the top and base so they stay whole. Place in a pan of cold water, bring to the boil and simmer for 5 minutes, then drain well.
2. Twist each sausage in half and cut. Peel the papery skin off the kidneys and cut them in half horizontally. Remove the tough cores and slice the remaining meat.
3. Melt the butter in a pan, add the kidney slices and sausages and fry quickly to brown. Lift out with a slotted spoon and keep warm. Sprinkle flour into pan, stir and cook for a minute.
4. Gradually blend in the wine and stock and bring to the boil, stirring continuously. Add the tomato puree and seasoning. Return the kidneys and sausages to the sauce with the onions.
5. Cover and simmer gently for 15 minutes. Taste and adjust the seasoning if necessary and serve in a warm serving dish.