Cumin and fiery serrano chile infuse this simple soup as it cooks, and a dollop of sour cream provides a refreshing foil for the spicy flavors. For less heat, seed the chile first or use a milder pepper, such as jalapeno. You can also omit the chile altogether, if your prefer. You can soak the beans before you go to bed, then wake up and put everything together in the crockpot. You will have a hearty, satisfying soup that is low in calories and fat waiting for you when you come home from work.