Either preheat your grill on high or turn the burners of your gas range to high. If you are using a grill, cook the peppers until they are blistered and charred on each side without crowding them together. If you are working indoors, place 1 to 2 over each burner so that they get contact with the flame. You will hear snapping and popping as the outer layer of skin is breached. Cook until blistered and charred on each side. Either way, this should take about 10 minutes. Transfer to a plate and cover with plastic wrap. Let stand 5 to 10 minutes.
Preheat your oven to 400 degrees.
When they have cooled some, skin each pepper by placing them inside of a piece of paper towel and rubbing vigorously. The skin should crumble right off the pepper. Skin all of the peppers, then trim the stems and remove the seeds with your hands. Rinsing them with water will wash away the peppers' oils, so avoid the faucet. Chop the peppers into smaller pieces and set aside.
Combine the tomatillos, jalepeños, garlic and onion in a large mixing bowl. Drizzle 1 tbsp. of oil over the veggies and toss with a large spoon. Transfer to a lined baking sheet and roast until soft, 30 to 45 minutes.
Heat the remaining oil over high heat until it shimmers. Add the pork and sear on all sides. Season the pork with salt and pepper. Cook in batches, transferring each batch to a paper towel lined plate when it is cooked.
Return all of the pork to the pan and add the roasted peppers and veggies. Add the chicken stock, cover the pan and cook 2 to 2 1/2 hours, or until the liquid has reduced and thickened, and the pork is tender.