Fry the sausages in the oil in a large frying pan, turning until they are golden on all sides and have released plenty of fat. Remove from the pan and keep warm. You can pour away some of the fat from the pan if preferred.
Fry the onion gently in the remaining sausage fat until soft and translucent. Add the vinegar and sugar and cook for a few minutes longer before adding the grapes. Increase the heat and cook, stirring constantly, for about 3 minutes until they are slightly soft and opaque. Check the balance of sweet and sharp and add more sugar and vinegar as needed.
Return the sausages to the pan and cook for a couple of minutes. If you can allow them to sit for 15 minutes before reheating briskly and serving.