Heat the oil in a large casserole dish over a medium-high heat and cook the onion and garlic for a few mins. Stir in the sausages, breaking them up into small pieces as you go, and cook for another 8-9 mins
Stir in the chopped tomatoes, stock, purée and most of the rosemary. Bring to a simmer and cook for another 8-10 mins or until the mixture is thickened. Season to taste.
Meanwhile, make up the polenta following pack instructions. Remove from the heat and stir in the cheese, with some seasoning.
Spoon the polenta onto 4 plates, then top with the ragout and a sprinkling of the remaining rosemary.