In large skillet, heat the olive oil over medium heat. Add onions and cook until softened, about 4-5 minutes. Stir in the garlic for about 30-60 seconds, until fragrant.
Add the onion/garlic mixture and the rest of the ingredients into a 5- or 6-quart slow cooker. Stir, cover, and cook on LOW for 9-11 hours (or on HIGH for 5-7 hours).
Taste and adjust seasonings, as desired.
Freezer Instructions
Freeze For Later: Cook the sauce and let it cool completely. Portion into to freezer bags, freezer containers, or mason jars, being careful to leave at least 1-inch of headroom at the top for expansion.Prepare From Frozen: Thaw the sauce using one of these safe thawing methods. Warm over medium-low to low heat on the stove, or use the microwave.
Notes/Tips
More Ways to Cook Marinara Sauce:
Slow Cooker High Setting - Cook it on high for 5-7 hours.
Stove Top: Simmer ingredients in a covered stock pot on the stove for 1 hour. I add 1 cup of water to the ingredients since it will lose more moisture this way.
Instant Pot: Depending on the size of your Instant Pot, you'll likely need to cut this recipe in half. In the Instant Pot use the Saute setting to cook the onions and garlic. Stir in the rest of the ingredients. Cook at high pressure for 10 minutes, then quick release the pressure.