Step 1Heat the oven to 350 degrees. Use a small sharp knife to make small 1-inch deep cuts all over the surface of the pork, and push one sliver of garlic into each slit. Season the pork with salt and pepper, rubbing the seasonings all over the meat.
Step 2In a large, heavy oven-proof casserole, heat the oil over medium-high heat. Add the pork and sear on all sides to a rich brown, 10 to 12 minutes. Remove the pork to a plate and set aside.
Step 3Add the onion to the casserole and reduce the heat to medium. Saute the onion until tender, 5 to 6 minutes, stirring occasionally. Stir in the minced garlic, the serrano chile and the tomatillos and continue to saute until the tomatillos are tender, about 5 minutes.
Step 4Add the chicken broth to deglaze the pan. Stir in the orange peel and 1 teaspoon of the oregano. Nestle the roast back with the vegetables and spoon some of the juices over the meat. Cover and roast 2 hours, until the pork is tender (it will pull apart easily). Remove the casserole from the oven.
Step 5Carefully lift the meat out of the casserole onto a carving board. Stir the basil, mint, remaining oregano, lime juice and a few grinds of black pepper into the sauce. Use a fork to break the meat apart into bite size pieces. Divide the pork evenly among six plates and spoon the tomatillo sauce evenly over the portions. Serve with grilled tortillas and/or rice.