1. Peel the onion and cut in half 2. Place the tomatoes, butter, onion halves and salt in a large saucepan and bring to a simmer over medium-high heat 3. Lower the heat to maintain a gentle simmer 4. Stir and crush the tomatoes lightly with the back of a fork occasionally 5. Cook until droplets of fat appear on the surface of the tomatoes, about 45 minutes 6. Remove and discard the onion, or use it for other purposes Server over hot pasta with Parmesan and black pepper