Soak black beans overnight in a bowl full of water. If you forget to soak, as I often do, then add the beans to boiling water, remove from heat and let soak for 2 hours. Once soaked, drain and rinse. More on cooking dried beans here.
Add the soaked and rinsed beans to large saucepan and cover with 2 inches of water. Add the chopped onion, crushed garlic, cumin, lemon juice, and salt in with the beans.
Bring to a boil, then reduce heat to medium and cover. Cook for about an hour.
After an hour, add the homemade chicken stock and the undrained can of Rotel diced tomatoes and green chilies. Let the soup simmer for another 30 minutes, or until the beans have softened.
Then add the cornstarch.