Cut an X on the bottom of each tomatillo and remove core. Drop into boiling water briefly, 15 to 20 seconds; drain and peel off husks. (If husks are hard to remove, drop tomatillos again into boiling water to cover, boiling about 1 minute, or until husks soften). Quarter tomatillos and place in a blender. Heat oil in a skillet over medium heat. Add onion and garlic and saute until limp. Add to blender, along with serranos, salt, pepper, chopped cilantro and nopalitos. Process until mixture is coarsely ground. Place pork in heavy pan or Dutch oven; add tomatillo mixture. Bring to a boil, reduce heat, cover pan and simmer slowly until pork cuts easily with a fork, about 1 to 1-1/2 hours. Serve garnished with cilantro sprigs and whole chilis.