Dried boletes harmonize well with the mustiness of the black beans, but you don't need to use your prized porcinis for this soup. This is an excellent recipe for using second-rate Bolete or Leccinum species like B. dryophilus, B. regius, L. manzanitae or L. insigne. The mushrooms simply add complexity to a soup that already has a substantial amount of flavor.
The day before cooking, begin by soaking the black beans in 2 quarts water overnight in a large kettle. The next day, drain the water, rinse and add: 12 cups water; 2 Tbspn. boullion, 1 tsp. salt, 1 cup brown rice, 1 bay leaf. Boil together for 45-60 minutes, until the beans are soft.
While the beans are cooking, chop the onions, garlic and cauliflower. Heat 1 Tbsp. olive oil in a frying pan, add the garlic and onions and cook until translucent. Near the end of the time required to cook the beans, add the onion mixture to the bean broth along with the chopped cauliflower. Cook for a few more minutes and remove from heat. Place the soup in a blender, a few cups at a time, and blend until smooth. Ladle into serving bowls, then dribble a dollop of sour cream on top before serving (thin the sour cream a bit with water if necessary.) Serves 4-6.