Remove the husks from the tomatillos and discard. Cut the tomatillos in half and arrange them on a foil-lined baking sheet. Add the unpeeled garlic and broil for 5-8 minutes or until tomatillos are deeply charred. If you have a gas range, fire roast the jalapenos and poblanos. If you do not have a gas range, add them to the baking sheet along with the tomatillos and garlic.
Place the jalapenos and poblanos in a bowl, and cover tightly with plastic wrap for 45 minutes. Peel, seed, and de-stem the peppers.
Peel the roasted garlic. Add the garlic, tomatillos, and peppers to a blender or food processor, working in batches if necessary, and pulse until smooth. Set the sauce aside.
Place a cast iron skillet over medium-high heat.
Add a little olive oil and heat until rippling. Working in batches, cook the pork, seasoning generously with salt and pepper, and ensuring all sides are golden. Place the pork in the bowl of a large crock pot or slow cooker and set on low heat. Add the sauce to cover the pork.
Meanwhile, using the same skillet you used for the pork, sautee the onions, scraping up any brown bits from the bottom of the pan. Once the onions are translucent, add the chicken stock and scrape the bottom of the pan again.
Add the onions and stock to the slow cooker. Add the remaining ingredients and mix well. Cook for 6-8 hours, or until pork is fork tender.