Empty the can of tomatoes into a pot. Add in 3 tablespoonfuls of water into the can and swirl to get all the tomatoes and juices and empty the water into the pot. Add in onion, garlic and butter.
Cover the pot and place over high heat. When the sauce has come to a boil, turn the heat down to low and simmer for 45 minutes to 1 hour, until the onion is extremely soft, stirring occasionally.
Remove from heat, use an immersion blender and blend until the sauce is smooth or until no large chunks of onion and garlic remains. If you don't want to blend the sauce, simply use a slotted spoon to remove the onion and garlic and if you are using whole tomatoes, crush the tomatoes against the side of the pot.
Taste the sauce - it's highly likely that you will not need anymore salt as the canned tomatoes are usually salted, but do add some if you think it's too bland.
Cover the pot and prepare your pasta. Reheat the sauce on medium-low heat until simmering if necessary.
NOTES
Variations: Add in sautéed sausages, or meatballs, or sautéed mushrooms, or a small handful of minced fresh herbs (like basil and parsley), or even a splash of cream.
Freezer Meal: Freeze the sauce in small containers (1/2 cup per serving). Thaw the sauce before using.