Soak beans overnight in 2 quarts water. (Alternatively, place beans and 2 quarts water in a large pot. Bring to a boil. Boil for 2 minutes. Remove from heat and let stand for 1 hour.) Drain beans, discarding soaking liquid.
Meanwhile, combine oil, mustard seeds, chili powder, cumin and cardamom in a 5- to 6-quart Dutch oven. Place over high heat and stir until the spices sizzle, about 30 seconds. Add onions, mushrooms, tomatillos and 1/4 cup water. Cover and cook, stirring occasionally, until vegetables are juicy, 5 to 7 minutes. Uncover and stir often until the juices evaporate and the vegetables are lightly browned, 10 to 15 minutes. Add broth, tomato paste and chipotles (with sauce); mix well.
Place the beans in a 5- to 6-quart slow cooker. Pour the hot mixture over the beans. Turn heat to high. Put the lid on and cook until the beans are creamy to bite, 5 to 8 hours.
To serve, ladle the chili into bowls. Garnish each serving with cheese, a dollop of sour cream and a sprinkling of cilantro. Pass lime wedges at the table.Stovetop method: Total: 4 1/2 hoursIn Step 2, increase broth to 8 1/2 cups. Omit Step 3. Add the beans to the Dutch oven; cover and simmer the chili gently over low heat, stirring occasionally, until beans are creamy to bite, about 3 hours.
Popular in Mexican cooking, the tomatillo looks like a small green tomato with a papery husk.Chipotle peppers are smoked jalapenos with a fiery taste, canned in adobo sauce.Mushroom broth can be found in the natural-foods section of large supermarkets and in natural-foods stores.