Step 1Place the nixtamal in a colander and rinse thoroughly. Pour it into a large Dutch oven or a heavy lidded pot, add 16 cups water and bring to a boil. Boil gently, loosely covered, 2 hours, or until tender. Do not allow the mixture to boil dry. Enough water should remain at the end of the cooking time to keep the nixtamal moist.
Step 2Meanwhile, wash the chicken. Place it in a heavy lidded pot with 8 cups water and 2 teaspoons salt. Bring it to a boil, reduce the heat, cover and simmer 1 hour. Strain and measure the broth, adding water if necessary to equal 8 cups. Shred the meat, discarding the skin and bones. Set aside the broth and the chicken.
Step 3When the nixtamal is tender, remove it to a bowl. Wash the pot, then return the cooked nixtamal to the pot.
Step 4Wash the poblano chiles and discard the stems and seeds. Cut into chunks. Wash the cilantro and cut off the tips of the stems. Place the chiles and cilantro in a blender jar and add 1 cup water. Blend until thoroughly pureed.
Step 5Slice 1 onion.
Step 6Add the chile-cilantro puree, the chicken broth, the sliced onion, the garlic cloves and 4 1/2 teaspoons salt to the nixtamal. Stir to combine. Bring to a boil, reduce the heat and simmer, covered, 30 minutes.
Step 7Add the chicken and cook 15 more minutes.
Step 8Dice the remaining onion and place it in a small bowl. Arrange the lettuce, serrano chiles, lime wedges and oregano in separate bowls, and the crisp tortillas in a basket or bowl.
Step 9To serve, ladle the pozole into large soup bowls and add lettuce, chile slices, lime juice and oregano as desired, with tortillas to accompany.