This versatile Italian tomato sauce is inspired by Marcella Hazan’s classic.
Peel the onion and quarter through the root. Put the tomatoes, onion, butter and salt in a medium-sized, heavy saucepan over medium heat. When the tomatoes start to bubble, reduce the heat to simmer. Cook for 45 minutes, stirring occasionally until the tomatoes have thickened and the sauce looks shiny.
Allow the sauce to cool slightly then purée with a stick blender. Portion up into six containers and pop in the freezer for later use.