Check the tomatoes for any bad spots and cut away any impurities.

Boil the tomatoes for three minutes or until they start to split and peel.

Remove the tomatoes from the boiling water and place them in ice-cold water to cool off.

Once cooled, peel off the skin and cut out the core with a knife.

In a food processor, add the onions, garlic cloves, fresh basil, salt, and any additional spices like parsley and oregano. Process until it forms a thick paste.

Heat olive oil in a large pot (you might need two pots). Add the onion, garlic, and basil mixture to the pot and cook until fragrant.

Add the peeled tomatoes to the food processor in batches. Blend until it reaches a smooth consistency similar to a thick smoothie, then transfer to the pot.

Bring the sauce to a boil, then reduce the heat to low or simmer.

Add the tomato puree to thicken the sauce. If you prefer it thicker, you can add more tomato puree.

Let it simmer for anywhere between 30 minutes to 2 hours, stirring occasionally.

If you like a smoother sauce, use an immersion blender for 2-3 minutes after a few hours of simmering.