Bring the stock to the boil with the rosemary. Stir in the polenta, reduce the heat, simmer over a very low heat for approx. 50 mins., stirring occasionally, to form a thick paste. Stir in the Parmesan and butter, season.
Heat the oil in a frying pan, fry the sausages on a medium heat for approx. 8 mins. on each side and serve on the polenta.