Another great recipe from Cook's Illustrated. This is a nice compromise between the thin-watery soup and the thick-chunky version. Be sure to offer a garnish or two; they not only add taste, but texture and color to the soup.
Place bean, ham, bay leaves, water and baking soda in large saucepan with tight-fitting lid.
Bring to boil over medium-high heat; using large spoon, skim scum as it rises to surface.
Stir in salt, reduce heat to low, cover, and simmer briskly until beans are tender, 1 1/2 to 2 1/2 hours (if necessary, add additional cup of water and continue simmering until beans are tender); do not drain beans.
Discard bay leaves.
Remove ham steak (which has taken on a weird black bean color), cut into 1/4 inch cubes and set aside.
For the Soup:
Heat oil in 8 quart dutch oven over medium heat until shimmering but not smoking; add onions, carrot, celery and salt and cook, stirring occasionally, until vegetables are soft and lightly browned, 12-15 minutes.
Reduce heat to medium-low and add garlic and cumin; cook, stirring constantly, until fragrant, about 3 minutes.
Stir in beans, beans cooking liquid, chipotle chili, adobo sauce and chicken broth.
Increase heat to medium-high and bring to boil, then reduce heat to low and simmer, uncovered, stirring occasionally, to blend flavors, about 30 minutes.
To finish the soup: Ladle 1 1/2 cups beans and 2 cups liquid into food processor or blender; process until smooth, and return to pot.
Stir together cornstarch and water in small bowl until combined, then gradually stir about half of the cornstarch mixture into the soup; bring to boil over medium heat, stirring occasionally, to fully thicken.
If soup is still thinner than desired once boiling, stir remaining cornstarch mixture to recombine and gradually stir mixture into soup, return to boil to fully thicken.
Remove from heat and stir in lime juice and reserved ham.