
Open the 4 cans of whole peeled tomatoes, drain them, reserving the juice, then run the tomatoes through a food mill to crush, or pulse quickly in a blender until smooth. Add the crushed tomatoes back to the juice and set aside.
In a heavy bottom sauce pot over medium heat, add the onions and oil and simmer onions about 2 minutes until softened, then add the slivered garlic and sauté until garlic starts to brown, about 1 more minute. Quickly add the crushed tomatoes to stop the garlic from browning, season with salt and pepper to taste, then bring to a low simmer.
Simmer slowly for about 30–40 minutes, stirring often with a wooden spoon, and sweeten with the teaspoon of sugar, if using. About 5 minutes before turning off the flame, stir in the fresh basil and parsley, and taste the sauce for seasoning. If you prefer a hot sauce, add some crushed red pepper flakes to taste.