Combine polenta, water, and salt in a saucepan. Place over moderately high heat and bring to a boil, whisking every so often. Turn down to a simmer and simmer gently, (partially covered with a lid) for 15 minutes. Stir in the butter and cheese, and season with salt and pepper to taste.
Meanwhile, make the sauce: Heat the oil over moderately low heat, add the onion and cook until softened. Add the garlic and cook 2 minutes more. Add all the remaining ingredients and simmer for 10 minutes or until thickened slightly. Serve over warm soft polenta and sprinkle each portion with some of the cheese.