1Rinse pinto beans. Place in large container or saucepan. Add enough cold water to cover (at least 4 cups). Refrigerate 4 hours or overnight. Drain well. Wrap Spice Mix in cheesecloth or coffee filter. Tie tightly with kitchen twine; set aside. Peel away papery outer layers of garlic peel, leaving head whole. Trim about 1/4 inch off the top of garlic to expose cloves, leaving root end intact; set aside.
2Place chuck roast, oxtail, stock and water in large saucepot. Cover; bring to a boil. Skim off any foam that rises to the top. Reduce heat to simmer. Stir in soaked pinto beans, spice mix sachet, garlic, onion, 1 of the plum tomatoes, Anaheim pepper, cilantro and Guajillo Chilies. Cover. Simmer 2 hours until beans and meat are tender.
3Remove onion, garlic, tomato, Anaheim pepper, Guajillo chilies and spice mix sachet from pot; set aside. Stir in hominy and 3 teaspoons of the salt. Cook 30 minutes longer or until hominy is tender.
4Meanwhile, squeeze garlic from the root end to remove cloves, discarding any remnants of the peel. Place cloves in blender container. Add onion, tomato, Anaheim pepper, Guajillos, remaining 2 tomatoes and remaining 1 teaspoon salt to blender container. Cover. Blend on high speed until completely smooth. (Discard spice mix sachet.)
5To serve, ladle soup into serving bowls. Top with prepared salsa. Garnish with chopped cilantro and serve with warm tortillas, if desired.