Combine the chopped onion, butter, and crushed tomatoes in a crockpot. Add water only if desired for a thinner soup. Cover with a lid and cook on LOW for 6 hours or on HIGH for 2 hours.

Uncover and puree with an immersion blender until completely smooth. (You can also transfer the soup to a blender and puree it that way.)

Serve now or return the soup to the crockpot. Hold on WARM setting until ready to serve.