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.
Recipe courtesy of Sara Moulton