Oaxacan Black Bean Soup

(Sopa de Frijoles Negros)
This soup really has a wonderful taste, and you don't soak the beans overnight. It can be a meal in itself, especially if the optional shrimp are added, but for a really fulfilling main course I suggest adding another six shrimp.

12 oz. (about 2 c.) dry black beans, cleaned
4 avocado leaves or 1 rib fresh fennel, roughly chopped
1/2 c. (4 OZ) chorizo sausage, casing removed
1 small white onion, diced
Salt, about 1 tsp.
1/2 c. vegetable oil, for frying
4 to 6 corn tortillas (preferably stale storebought ones), sliced into 1/8 in.-wide strips, for garnish
8 oz. (about 12) medium large shrimp, if desired, peeled
About 1/2 cup crumbled Mexican queso fresco or pressed, salted farmer's cheese, for garnish

Place the beans in a medium-size (4-quart) pot, cover with 6 cups water, remove any beans that float and heat slowly to a simmer.
If using avocado leaves, toast them briefly directly over a medium gas flame or on a hot griddle. Add the avocado leaves (or fennel), chorizo and onion to the beans, partially cover and simmer over medium-low heat, stirring occasionally, until the beans are fully tender, 1-1/2 to 2 hours. If you see the beans peeking up through the liquid, add hot water to cover them by 1/2 inch.
Use an immersion blender to coarsely puree the soup, or puree in batches in a food processor or loosely covered blender. Return it to the pot. Add enough water to thin to a medium-thick consistency. Taste and season with salt.
Heat the oil in a medium-size (8- to 9-inch) skillet over medium-high. When hot, add about 1/3 of the tortilla strips and fry, turning frequently, until they are crisp. Drain on paper towels. Fry the remaining strips in 2 batches.
Heat the soup to a boil, add the shrimp and cook until just done, about 2 minutes. Ladle into warm soup bowls, top with a few of the crisp tortilla strips and sprinkle with a little cheese.
Notes: The soup can be made and refrigerated several days ahead through the pureeing step. You'll likely need to thin the reheated soup before adding the (optional) shrimp. The tortilla strips keep a day or so wrapped in foil on the counter.
Don't try to take a shortcut by using canned beans. The result will be quite gray. Broken, good-quality chips could replace the fried tortilla strips.

Makes 6 servings.