Bring the water to a boil in a 6-quart saucepan. Pour the polenta in a thin stream into the boiling water and, whisking constantly, cook until it is as thick as porridge. Continue cooking, stirring with a wooden spoon until it is too thick to stir. Pour the polenta out onto a wooden cutting board.
In a 12 to 14-inch saute pan, heat the oil until it is smoking and add the onion. Cook until just softened, or 2 to 3 minutes, and add the sausage pieces. Cook until browned on all sides and drain the fat from the pan. Add the portobellos, rosemary and red wine and bring to a boil. Simmer 10 minutes covered, or until sausage is cooked. Slice pieces of polenta off the board with a knife and place in each of four warmed pasta bowls. Remove the sausage mixture from the heat, season, to taste, and pour over polenta. Serve immediately.
Recipe copyright 2001, Mario Batali. All Rights Reserved.