1 pound dried black beans
1 teaspoon cumin seed
1 teaspoon coriander seed
1 teaspoon roasted Mexican oregano
1/2 large onion, peeled, with root fibers attached
4 large cloves garlic, slightly mashed
2 jalape? chiles, cut lengthwise
1 bunch cilantro, tied
2 bay leaves
1/2 teaspoon dried thyme
1 1/2 teaspoons black peppercorns
6 quarts water
1/2 cup tomato pur?
1 1/2 teaspoons salt

Remove any foreign particles from the beans. Watch for small black stones that resemble the beans. Rinse beans in cold water three times, stirring by hand. Roast cumin, coriander and oregano together and crush or roughly grind them in a spice mill. Tie them up together with the onion, garlic, jalape?s, cilantro, bay leaves, thyme and peppercorns in cheesecloth and simmer in water for 15 minutes.

Add beans, tomato pur? and salt to the water and simmer very low for 3 hours, or until the beans are completely soft and just starting to fall apart. Add water as necessary. Remove the cheesecloth bag. To make a thicker soup, pur? 1 1/2 cups beans and stir them back into the soup.

Pour the soup into individual serving bowls and garnish with equal amounts of sour cream, chopped avocado, minced scallions, chopped cilantro and minced serrano chiles.

Serve with a lime wedge.