Coat a large saucepot with olive oil and add the pancetta. Bring the pot to a medium-high heat and cook the pancetta for 4 to 5 minutes. Add the onions, season generously with salt and stir to coat with olive oil. Cook the onions for 6 to 7 minutes stirring frequently. The onions should become very soft and aromatic but have no color. Add the garlic and cook for another 2 to 3 minutes stirring frequently.
Pass the tomatoes through the food mill. Be sure to pass all of the pulp through the holes leaving only the stems and the seeds, and be sure to scrape the pulp off of the bottom of the food mill. That's all of the big money stuff! Add the tomatoes to the pot and rinse out 1 of the empty tomato cans with water and add that water to the pot (about 2 to 3 cups). Season generously with salt and TASTE IT!!!! Tomatoes take a lot of salt. Season in baby steps and taste every step of the way. Cook the sauce for 2 to 3 hours, stirring occasionally and tasting frequently.
Use the sauce right away on pasta or for any other tomato sauce need. This sauce can also be cooled and stored in the refrigerator for a few days and freezes really well.