This recipe was in Rachel Roddy's 'Italian Kitchen' column in The Guardian a few weeks ago and I have been using it ever since I read it.
800g chopped tinned tomatoes
1/2 onion, peeled
pinch of salt
Put tomatoes in a pan. Add the butter and onion. You don't even need to chop the onion first, just pop it in. Bring to boil then simmer, uncovered, for about 40 minutes. Add salt to taste. That's it!
I have started to whizz the sauce up as the children prefer it free from tomato lumps. I whizz up the onion as well, which makes it nice and sweet. You could, of course, add other things to it, such as garlic and herbs, but it is still delicious just as it is.