Place the tomatoes, butter, and onion quarters in a large saucepan over medium heat. Bring to a simmer and lower heat to medium low. Continue to simmer for 45 minutes to 1 hour, stirring occasionally and crushing the tomatoes against the side of the pan.
Remove the sauce from heat and discard the onion. Taste the sauce and season with salt and pepper.
If desired, place the sauce in a blender and blend until smooth.
Serve the sauce over top of spaghetti or your favorite pasta shape.