For true Southwestern flavor, squeeze lime juice over each portion of soup just before serving and place tortilla chips alongside. Buy the cooked ham from the deli counter, or leave it out altogether for a vegetarian version.
In a large pot, cook ham over mediumhigh heat until browned on both sides, 4 to 5 minutes. Transfer to a cutting board.
Reduce heat to medium, and add oil. Cook onion, stirring frequently, until ightly browned, 2 to 3 minutes. Add jalapeño, garlic, cumin, and oregano; cook, stirring, until fragrant and garlic is tender, 1 to 2 minutes. Add beans and the water; simmer 10 minutes.
Remove from heat; stir in cilantro leaves. Using an immersion or regular blender (see page 363; work in batches so as not to fill jar of blender more than halfway), puree mixture until smooth. Add more water to thin soup, if needed. At this point, soup can be refrigerated up to 3 days in an airtight container; let cool completely before storing. (Refrigerate ham in a separate container.) Reheat gently before serving.
To serve, ladle soup into bowls. Cut ham into 1/2-inch cubes; sprinkle over soup. Garnish with finely chopped jalapeño and cilantro sprigs, as desired.