Defrost bags of frozen tomatoes; takes about 8-24 hours
Remove skin from tomatoes.
Process tomatoes through a food mill (to puree flesh of tomatoes and remove seeds and any remaining tomato skin).
Separately blend onions, garlic and olive oil in a small blender to make a paste.
Add onion mixture to a large pot and simmer for 5 minutes. Add tomato puree to pot and mix in. Bring to a boil then lower temperature to LOW and simmer for 4-6 hours.
Once sauce has reduced to about half its volume, add tomato paste, salt, sugar and basil. Stir, and check taste and thickness. Adjust seasonings to taste and continue to cook to desired thickness.
Can or freeze sauce in jars for later use.