Fry the butter over a medium heat in a large lidded pan and once melted, add the chopped onions and fry for a few minutes until softened.
Add the minced garlic and cook for another minute or two, before adding the tinned tomatoes and tomato puree.
Stir until well combined and bring to a simmer. Pop the lid on and let the sauce reduce for 30-45 minutes. Keep checking on it from time to time and use splashes of water to stop the sauce from sticking. I just fill the empty tomato can with tap water and use as much as I need.
Season to taste, and that really is it. It’s the cooking time that completely changes the flavour of the tomatoes. This sauce is perfect through pasta, or a great base for meat and fish.