1Put the tomatoes, butter and onion in a pot over medium heat. Once it starts to bubble, turn the heat down to medium-low and simmer for about 45 minutes. Put a lid on if you don't want any red spatters in the vicinity of the pot. Stir occasionally, squishing the tomatoes against the side of the pot with your spoon.
2Fish out the onion halves (or leave it in and puree the lot) and if you like, blend until smooth with a hand-held immersion blender. Serve hot, ladled over spaghetti.