First, in a large Dutch oven or pot, add oil, minced garlic and chopped onion. Stir and cook on med/high heat for 3-4 minutes, or until the onion is translucent.
Next, slowly (to avoid splatters) add tomato sauce to the oil mixture. It will look oily and that's o.k. Stir it so the oil is combined with the sauce.
Then, add the parsley and pepper. If you choose to add salt, it's best to add it at the END of the cooking time so you can judge if it is to your liking as is. The canned tomatoes have a good bit of salt already.
Next, bring sauce to a boil, cover and cook on low heat for 2.5 hours. Stir (to the bottom of the pot) every 15 minutes to avoid sticking and burning. Using a heat diffuser, works well.
Last, allow sauce to cool completely before putting in refrigerator or freezing.