In a large saute pan or skillet, heat the olive oil over medium heat. When hot and just starting to smoke, add onions and peppers and saute for about 4-6 minutes; stirring occasionally until the onions are translucent. After 5 minutes, add the garlic and continue to cook an additional minute longer to release the garlic oils. Immediately transfer mix into the slow cooker. De-glaze the pan with the red wine, add liquid to crock pot.

Wash tomatoes thoroughly. Cut into quarters or even smaller pieces if you are using large tomatoes. Remove the hard center top core. Add the tomatoes and tomato paste to the slow cooker. Stir in the balsamic vinegar and mix. Add the spices, stir, and cook for 8 hours. Keep lid on for the entire cooking time.

After 8 hours, open the lid and use an immersion blender to puree the sauce. If you do not have a hand blender you can puree small batches in a blender or food processor. Salt or pepper additionally if needed. Be careful to not over salt as this sauce will most likely be used to season already salted food.

Return to the slow cooker, set on high heat and simmer uncovered until the desired thickness. Transfer the sauce to sterilized canning jars, carefully wipe the rims and seal. If you will use the sauce within 2 months, just refrigerate.

If you plan a longer storage, do a full water bath. In a large pot, completely submerge the jars and boil for 30 minutes. Remove from pot and allow to cool to room temperature. Alternatively you can freeze the sauce in freezer bags.