Parboil the hot sausage for about 10 min. Add the grapes and the balsamic vineger to an oval baking dish. Place the sausages on top of the grapes. Bake the dish at the highest oven setting for 8 min. and turn, then bake for another 8 min. After the sausage are browned, remove them, mash some of the grapes and heat them with the pan juices until boiling. Then add the butter and whisk into a sauce. Serve the sausages with the sauce and mashed potatoes.