Place the contents of the can of tomatoes and the onion into a food processor and pulse until the onion is finely diced.

Pour the tomatoes and onion into a 4-quart slow cooker. Add vegetable broth and chili powder. Stir, and cook on low for 6-8 hours, or until onion is translucent. Stir in peanut butter.

Cover and cook on high for 30-45 minutes. Stir well and season with salt and pepper, to taste. Serve in a wide-mouthed bowl with a dollop of sour cream in the center of each bowl, if desired.