Using a sharp knife, cut a ¼-inch deep X on one end of each tomato. Place the tomatoes in a large, freezer-safe bag, and freeze overnight.
The next day, move the tomatoes from the freezer bag to a colander, and let tomatoes defrost and drain excess water.
Remove tomato peels, which should slide off easily. Core the tomatoes, removing any excess seeds. Chop tomatoes into smaller pieces.
Dice the large sweet onion and mince fresh garlic. Add to a large pot with the olive oil in the bottom, and cook for 2 to 3 minutes.
Add the tomatoes, salt, and sugar to the pot. Reduce heat to low and simmer for 2 hours, stirring occasionally and mashing tomatoes until the sauce is no longer watery.
Taste and add additional seasoning if necessary.
Transfer sauce to a blender, and blend until the sauce reaches your desired consistency.
Stir in basil before serving.