In a bowl, combine the chili powder, ground cumin and oregano. Rub spice mixture over the pork roast. Place seasoned pork in a slow cooker. Add the beans, garlic, green chiles, HERDEZ® Salsa Verde and chicken broth. Cover and cook on HIGH for 6-8 hours or until meat is tender. Remove meat from slow cooker and shred. Return meat to the slow cooker and continue to cook until mixture is thickened. Serve mixture on tostada shells or in tortillas. Top with shredded lettuce and HERDEZ® Salsa Casera