Place onion, bell pepper and mushrooms in slow cooker.
Add tomatoes to fill Crockpot to just under the rim.
Top vegetables with tomato paste, garlic, sugar, salt , spices, olive oil and Worcestershire.
Cook on high for 4-5 hours. (Stirring after 2 hours)
In 2 cup batches, pulse in blender to desired smoothness. (Probably no more than 30 seconds per batch.)
Pour sauce back into Crockpot and add Parmesan, if using.
Allow sauce to continue cooking for 1-3 more hours.
Fill 2 cup freezer containers with sauce and allow to cool on counter for 20 minutes before placing freezer for later use.