Heat the olive oil in a saucepan. Add the onion and cook for 4 minutes until softened. Add the garlic and oregano, cook for 1 minute.
Pour in the chopped tomatoes and sugar. Season with salt and pepper. Simmer for 15 minutes until thickened.
Meanwhile, bring the vegetable stock to the boil in a separate pan. Slowly pour in the polenta in a thin stream, whisking constantly to avoid lumps.
Cook the polenta for 5 minutes, stirring frequently, until it pulls away from the sides of the pan. Remove from the heat and stir in the butter and grated cheese.
Taste the tomato sauce and adjust the seasoning. Tear in the fresh basil if using.
Spoon the creamy polenta into bowls and top with the tomato sauce. Serve immediately.