This recipe by Chef Rick Mahan is a rather spicy soup. If a milder version is desired, reduce the amount of Serrano chilies and chili powder.
1 pound black beans, washed and soaked for 2 hours
2 tablespoons vegetable oil
1 cup diced onion
1/2 cup diced carrot
1/2 cup diced celery
1 to 3 serrano chilies, seeded and minced 1/2 cup diced red bell pepper
2 teaspoons minced garlic
2 tablespoons chili powder
2 tablespoons ground cumin
2 teaspoons dried Mexican oregano
2 teaspoons salt or to taste
1 teaspoon black pepper
1 teaspoon sugar
1/4 cup tomato paste
2 1/2 quarts water
Garnish: sour cream, salsa and cilantro sprigs