Preheat oven to 350 degrees.

Cut the pork into 6-8 pieces. Melt the lard in a heavy Dutch oven just large enough to hold the meat in one layer. Brown the meat on all sides and season with salt. 

Squeeze the orange over the meat and toss the rind into the pan. Pour the milk over the meat and add enough water to almost cover the meat. Sprinkle with pepper. Cover tightly with aluminum foil (or lid of Dutch oven) and bake for 2 hours, until the meat is fork tender. 

To finish, remove the meat to a cutting board and break into largish pieces. Reserve a cup of the liquid and discard the remaining orange rinds. Place pulled meat in a roasting pan and add the cup of the reserved liquid back into the pan.

Roast, uncovered, until most of the liquid has evaporated, about 20 minutes, or until the meat is brown and crisp on the edges. 

To assemble, fill warm corn tortillas with shredded pork. Top with avocado, mango salsa, pork rind crumbles, onion and cilantro.