Place the tomatoes, butter, onion halves, and a pinch of salt in a large saucepan and bring to a simmer over medium-high heat. Lower the heat to maintain a gentle simmer and cook, stirring and crushing the tomatoes lightly with the back of a spoon occasionally, until droplets of fat appear on the surface of the tomatoes, about 45 minutes. Remove and discard the onion. Serve over hot pasta with Parmesan and black pepper, if desired.