Begin by gathering all your ingredients. Open the can of whole tomatoes and set aside. You might want to crush them slightly with your hands or a fork to break them up, but this is optional.
In a large pot or Dutch oven, add the entire can of tomatoes along with the juice, the butter, and the halved onion.
Place the pot over medium heat. As it heats up, bring the mixture to a gentle simmer, stirring occasionally. Allow it to simmer uncovered for about 45 minutes.
Once the sauce has thickened and the flavors have developed, remove the onion halves from the sauce. You can either discard them or chop them finely and return them to the sauce for added texture.
Taste the sauce and season with salt if desired. Remember, the butter adds a rich flavor, so be mindful of how much salt you add.
Toss your cooked pasta of choice with the sauce, ensuring every strand is coated with the luscious tomato and butter mixture. Serve with grated Parmesan cheese if you like, and enjoy the magic of this simple yet spectacular sauce.