Use a 6-quart slow cooker. In a large skillet over medium heat, brown the turkey and onion until the meat is no longer pink and the onion is translucent. Pour the meat mixture into your slow cooker insert and add the kidney beans and the tomatoes with chilies. Stir in the chili powder (the mixture will be quite thick). Cover and cook on low for 8 hours or on high for about 5 hours. Serve with a dollop of sour cream and a bit of shredded cheese, if desired.