Step 1Cut open the cascabel chiles and remove the seeds. Heat the oil in a small skillet over low heat. Add the chiles and cook, stirring, until fragrant, 3 to 4 minutes. Place in a bowl, cover with water and soak overnight. Before using, drain the chiles, discarding the water; remove the stems and any remaining seeds.
Step 2Place the nixtamal in a colander and rinse thoroughly. Place in a Dutch oven or heavy lidded pot and add the water. Slice 1 onion and add it to the pot along with the garlic cloves. Bring the mixture to a boil and boil gently, covered, 1 1/2 hours.
Step 3Meanwhile, place the tomatoes in a pan of boiling water and cook 2 minutes. Drain and allow to cool, then peel. Place the tomatoes and the chiles in a blender and puree. Set aside.
Step 4Remove the excess fat from the pork. The meat can remain in large chunks. When the nixtamal has boiled 1 1/2 hours, add the pork, the tomato-chile mixture and the salt. Cover and boil gently 1 1/2 hours longer.
Step 5Chop the 2 remaining onions. Arrange the chopped onions, lettuce, radishes, oregano, ground chile and lime wedges in serving bowls.
Step 6To serve, ladle the pozole into large soup bowls. Add the garnishes and squeeze in lime juice as desired, with tortillas to accompany.