Put the tomatoes along with their juices, the butter and the halved onion into a pan.
Add a good pinch of salt and then simmer. Cook gently for 45 minutes.
Stir occasionally and mash any large chunks of tomato with your wooden spoon.
Remove from the heat, taste and add more salt if needed.
Marcella says to remove and discard the onion. By discard I think she merely means to remove it from the sauce to truly appreciate the simplicity of just having the tomatoes.
Truly though, I for one never waste the onion and like to fry the onion separately with some butter to have served with steak, or smothered on a sandwich…it’s heavenly.