01 -
Place canned or prepared fresh tomatoes, onion halves, unsalted butter, and kosher salt in a medium saucepan.
02 -
Simmer mixture uncovered over low heat for 45 to 60 minutes. Stir periodically, gently mashing the tomatoes with a spoon as they cook.
03 -
Discard the onion halves. Serve sauce immediately or allow to cool and refrigerate for later use.