Heat the butter in a large heavy bottomed non-reactive saucepan and saute the shallots. Add the Arborio rice and cook for at least 3 minutes.
Add 1 cup of the stock and the wine, if desired. Add the mushrooms and sprigs of rosmary. Stir until the liquid is absorbed. Continue to add the stock, about 1 cup at a time, stirring until each addition is absorbed. Towards the end, check the consistency of the rice by biting a grain with your teeth. It should be cooked, but slightly firm in the middle (al dente). As each batch of rice differs slightly from the others, you may have to use a little more (or less) liquid.
Fold the Parmesan cheese and, if desired the butter or crème fraîche. Serve immediately.