Gather the ingredients.
Place tomatoes in a medium saucepan; add onion, and stir in butter and salt. Cook over medium-low, uncovered, stirring occasionally and mashing any large pieces of tomato using the back of a wooden spoon, until fat floats free from the tomatoes and is incorporated into the sauce as it slightly thickens, 1 hour to 1 hour and 10 minutes. Season with additional salt to taste. Remove and discard onion.