Heat a dash of oil in a non-stick frying pan. Brown the meat for approx. 3 mins. per batch, remove, season. Reduce the heat, sauté the onions, garlic and rosemary in the same pan. Add the squash, mushrooms and tomato puree, cook briefly. Pour in the wine and reduce almost completely. Pour in the stock and balsamic, bring to the boil, reduce the heat, return the meat to the pan, cover and simmer for approx. 15 mins.
Bring the water to the boil, season with salt, stir in the polenta, reduce the heat, simmer over a low heat for approx. 4 mins., stirring constantly to form a thick puree. Stir in the mascarpone, season. Serve the ragout with the polenta.