Using a food processor, puree the onion, carrots, celery and garlic together.
Heat 2 tbsp of olive oil in a pan and cook the onion, carrots, celery and garlic until soft.
Add all ingredients to the slow cooker and stir together well. Set the slow cooker to 8 hours on low and let the sauce cook.
Once cooked, let it cool. If you like a smoother sauce pour the mixture into a blender and make it a consistency that suits your taste. This recipe will make about 16 cups of sauce, which makes it great to put in containers and freeze.