In a large nonreactive pot over medium heat, add oil and heat. If using fatback, cut it into small pieces and add to the pan. Saute for 5 minutes or until all fat has been rendered. Remove and discard fatback.
Add the onion, and saute for 3 minutes or until translucent, and just becoming brown. Stir in garlic, and saute for 30 seconds or until soft. Stir in tomatoes, reserved juice, and salt. Raise heat, and bring to a boil. Immediately reduce heat to a very low simmer and cook for about 1 hour, or until flavors have combined and sauce is slightly thickened. If you prefer a thicker sauce, cook for an additional 15 minutes.