Process the plum tomatoes with their juices in a food processor until finely chopped (not pureed but not too chunky); transfer to a large heavy saucepan. Stir in the tomato puree and place the slivers of butter on top. Heat over low heat, stirring often, until the butter has melted and the sauce is slightly thickened. Season with a little salt and pepper. This sauce can be stored in the refrigerator up to 1 week or frozen.