Heat the oil in a large frying pan or flameproof casserole and fry the sausages turning frequently, for about 10 minutes, until browned. Drain to a plate.
Add the onions and sugar to the pan and cook gently, stirring frequently, until lightly browned. Return the sausages to the pan with the sweet potatoes, sage leaves and a little seasoning.
Cover the pan with a lid or foil and cook over a very gentle heat for about 25 minutes until the potatoes are tender.
Drizzle with the vinegar and check the seasoning before serving.