In a 4-qt. saucepan combine 61/2 cups of the tomatoes, onion, butter, and 11/4 tsp. kosher salt over medium heat. Once tomatoes begin to bubble, reduce heat to a simmer. Cook 30 minutes, stirring every 10 to 15 minutes. Stir in remaining tomatoes; cook 15 minutes more or until butter has separated from the tomatoes and most of the liquid has evaporated. Serve over cooked polenta. Top with fresh oregano.