Heat oven to 300 degrees. Using a sharp knife score the pork skin in a crosshatch pattern, making 2-inch deep incisions an inch or so apart. Mash the garlic, oregano, ancho chili, salt and black pepper together in a mortar and pestle.
In a large bowl mix the orange, lime, stock, onions, and olive oil, and set aside.
Rub the garlic mixture all over the pork and into the incisions until well distributed. Put pork in a roasting pan and cover with citrus mixture. Cook pork, turning every hour or so and basting with pan juices, until meat is very tender, up to 5 hours (a 4-pound shoulder may be done in 3 hours). Turn the pork skin-side up, raise the oven temperature to 400, and roast until crisp, an additional 10-15 minutes.
Let meat rest for 30 minutes before slicing.
Serve with fresh tortillas, lime and fresh cilantro, and Grandma Andrea’s Garlic Sauce.
TO MAKE GRANDMA ANDREA'S GARLIC SAUCE:
In a “pilon” (mortar and pestle), mash garlic gloves and kosher salt together until a chunky paste forms.
Add black pepper and oregano and transfer into a clean 1.5 quart mason jar.
Add the olive oil and apple cider vinegar and seal the jar.
Shake well to combine and store in the refrigerator for up to two weeks.