Heat the olive oil in a large frying pan set over a medium heat, tip in the sausages, then cook for 10 mins, turning every so often. Stir in the onions, then leave to cook for 5 mins more until the sausages are browned and the onions softened.
Tip in the garlic, fennel seeds and grapes and cook for 5 more mins, stirring often, until the grapes are starting to soften. Pour over the vinegar and swirl around the pan. Cook for a few mins more until the onions are sticky and the sausages cooked through. Serve with soft polenta or mashed potato.