In a skillet melt the butter and saute the bell pepper, onion, garlic, salt and pepper until the vegetables are translucent and very soft. Pour into the crock pot.
Add the cooked butternut squash, tomato paste and water to the crock pot. Sprinkle the sugar over the top. Stir to mix well.
Place the cover on the slow cooker and turn the heat to low.
Slow cook for 8 hours if using cubed butternut squash. If using mashed butternut squash cook for 6 hours.
Once the sauce has simmered and has thickened if you used cubed butternut squash scoop the solids from the slow cooker and puree in a blender. Pour the puree back into the crock pot and mix well.
If the sauce is not thick enough for your liking turn the crock pot heat up to high and prop the lid so it is slighlty ajar and there is room for the steam to exscape. Continue cooking for another hour or so or until the sauce has thickened to your liking.
Turn the slow cooker off and allow the sauce to cool.
Once cool, if not using immediately transfer the butternut squash sauce to storage containers. Can be frozen in portion size to use another day.