Rinse pork shanks and place in a saucepan with 2 quarts of water, a quarter of an onion, 2 teaspoons of salt and a garlic clove. Bring to a boil, reduce the heat to a simmer and cook for 2 ½ hours until meat is tender. Cut the chicken legs into drumsticks and thighs, rinse and place in another saucepan. Cover with 5 cups of water. Add the remaining quarter of an onion, 2 teaspoons of salt and a garlic clove. Bring to a boil, reduce to a simmer and cook for 40 minutes until chicken is tender. Remove chicken and pork from the broth and allow to cool. Strain the both broths separately and reserve.
Place the corn in a saucepan with the strained chicken broth and simmer for 20 minutes. Add the pork broth, reserving 2 cups for pureeing the remaining ingredients.
Place the tomatillos in a blender jar with the reserved pork broth. Remove the stems and seeds from the pasilla and serrano chiles and add to the blender jar along with the cilantro, avocado, half of the lettuce, onion, garlic and salt. Puree on high until smooth. Add to the saucepan with the corn. Tear the chicken and pork into bite-size pieces and simmer for 20 minutes to combine flavors.
To prepare the garnish for the posole, heat a small skillet over high heat with the vegetable oil and arbol chiles. Toast in the oil until they darken in color. Remove from the oil and set on a plate lined with a paper towel. Season lightly with salt. Shred the remaining lettuce.
To serve, ladle the soup into large soup bowls. Garnish each bowl with some shredded lettuce, minced onion, sliced radish and a crumbled arbol chile over top. Serve with a lime wedge and tostadas on the side.
Enjoy with a glass of Cakebread Cellars Chardonnay.