Heat the tomatoes, onion and butter in a heavy saucepan over medium heat. Bring to a simmer, then turn down the heat to low. Allow to simmer at a steady pace for about 45 minutes. Stir occasionally, crushing the tomatoes against the side of the pot with a wooden spoon. Remove from heat, discard the onion and salt to taste (optional, I do not add any). Serve warm over pasta, topped with sliced Parmesan cheese.