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. Keep the polenta warm while you prepare the sausage mixture.

In a 12 to 14 inch sauté pan, heat the oil until it is smoking and add the onion. Cook until just softened, about 2-3 minutes, then add the sausage pieces. Cook until browned on all sides then drain the fat from the pan.

Add the portobello slices, rosemary, red wine, and tomato sauce and bring to a boil. Simmer 10 minutes covered, or until the sausage is cooked through. Spoon equal amounts of polenta into each of 4 warmed pasta bowls. Remove the sausage mixture from the heat and spoon over the polenta. Finish each serving with a dollop of crème fraiche, season to taste, and serve immediately.