Pour water in a 6 quart sauce pan and bring to a boil. Pour polenta in a thin stream into boiling water and, whisking constantly, cook until as thick as porridge. Continue cooking, stirring with a wooden spoon until too thick to stir. Pour polenta out on to a wooden cutting board. In a 12 to 14-inch saute pan, heat oil until smoking and add onion. Cook until just softened, or 2 to 3 minutes, and add sausage pieces. Cook until browned on all sides and drain fat from pan. Add portobellos, rosemary and red wine and bring to a boil. Simmer 10 minutes covered, or until sausage is cooked. Slice pieces of polenta off board with a knife and place in bowls. Remove sausage mixture from heat, stir in sour cream, season to taste and pour over polenta. Serve immediately.