Place blended tomatoes, fresh basil (if using dried basil-add in the last hour), onions, garlic, and oil in the crock-pot, cover with the lid, and cook on high for 8 hours (crushed tomatoes may cook quicker, so check after 4 hours and every hour after or cook them on low and check after 6-8 hours).

In the last hour of cooking stir in the Italian seasonings, salt, pepper, and cook with the lid off or partially open to thicken the sauce.

Continue cooking sauce as needed with the lid slightly ajar until desired consistency is reached. This might take hours depending on how watery your tomatoes were (I've cooked overnight in this step before). To expedite this process you can just boil your sauce in a pan on the stove top, too if you need the sauce faster. I've boiled part of it so I could use it then and there, and kept the rest cooking in the crock-pot for later use.

Cool in glass containers leaving an inch of room at the top of the jar or in freezer bags.

Store in the refrigerator up to five days or in the freezer.