To cook the tomatoes: Check for any “impurities” (terrible bruises, moldy spots) of the tomatoes, and chop off those parts to discard. Boil the tomatoes for three minutes, or until they start to split and peel. Take the tomatoes out and place them in ice cold water to cool off. Once cool, peel off the skin. Use a knife to cut out the core.
In a food processor add the onions, garlic cloves, fresh basil, salt, and any other spices you would like (I add parsley and oregano to mine). Process until it’s a thick paste.
Heat up olive oil in a large pot (you probably will need to split this up into two pots). Add the onion/garlic/basil mixture to the pot and cook until fragrant.
Add the tomatoes to the food processor. You’ll probably do this a few times to get all the tomatoes through. Once they are completely blended through (almost like a thick smoothie) add to the pot.
Bring the sauce to a boil. Once boiling, turn the stove on “low” or simmer
Add the tomato puree, which makes the sauce thick. If you want it even thicker, add another can or so of tomato puree until it is thick to your likeness.
Let it sit for a while! Make sure to stir every now and then. You can let it simmer anywhere between 30 minutes to 2 hours.
If you want a smoother sauce, use an immersion blender after a few hours of simmering. The blending shouldn’t take long, maybe 2 or 3 minutes.