Bring a large pot of water to a boil. Cut a large X on the bottom of each tomato. Set up a large bowl with 2 cups of ice and fill ½ way with cold water.

Place the tomatoes into the boiling water and cook for approximately 2-3 minutes until the skin begins to split. Remove the tomatoes with a slotted spoon and transfer them directly into the ice bowl. Let the tomatoes sit for a few minutes to cool before transferring them to a cutting board. Cut the tomatoes into quarters and remove the core from each tomato.

In a large pot on medium heat add the olive oil and once hot add in the onions and garlic, sprinkle with salt and leave to saute for 4-5 minutes until the onion begins to soften.

Add the chopped tomatoes and butter to the pot bring to a boil and then reduce the heat to low and leave the sauce to simmer for 2 ½ hours stirring occasionally

Remove the sauce from the heat and use an immersion blender to blend the sauce for a few seconds into a thick consistency. Season with additional salt.