Basic Tomato Sauce

In a medium saucepan, heat the olive oil over moderate heat until shimmering. Add the onion and cook, stirring, until softened, about 8 minutes. Stir in the carrot, then add the tomatoes and season with salt. Simmer over moderately low heat, stirring occasionally, until the sauce has thickened, about 40 minutes. Remove from the heat and stir in the butter and oregano.