Begin by preparing your salsa verde: preheat the oven to 200°C, and line a medium-large baking sheet with foil.
Place the tomatillos, Anaheim peppers, onion and garlic into a large bowl, and drizzle with about 2 to 3 tablespoons of the oil; sprinkle generously with salt and pepper and toss to coat, then turn the vegetables out onto the baking sheet, and roast for 20 minutes.
After 20 minutes, reduce the oven temp to 180°C, and roast for 15 minutes more, until the veggies are charred, tender and aromatic; remove from oven and allow them to cool slightly, just enough so they can be handled.
Once slightly cooled, add the roasted veggies, along with all of their juices, to the bowl of a food processor, then add in the coriander and the agave; process for about 30 seconds or so until the mixture has a chunky-smooth texture, like that of salsa; spoon into a bowl and set aside for a moment.
To prepare the chili verde pork, combine the cumin, smoked paprika, ground coriander, chipotle powder, dried oregano, white and black pepper in a ramekin, and set aside 1 tablespoon of this to use as seasoning for the pork.
Place the pork into a large bowl, season generously with salt, and sprinkle over the 1 tablespoon of seasoning, and toss to coat well.
Place a large pot over high heat, and drizzle in about 3 tablespoons of oil; once very hot, add some of the seasoned pork to the pot (working in batches), and allow it to sear/brown on that first side, undisturbed, for about 4 minutes; then, stir, and allow the pork to brown for about 2 minutes more, then remove from the pot and spoon into a large bowl to hold; repeat with remaining pork until all is browned.
Next, to the same pot, add in the onion, and saute that for a few moments until it becomes softened and slightly browned; add in the remainder of the seasoning/spice mixture you prepared, along with the garlic, and stir to combine for about 30 seconds, or just until aromatic.
Pour in your salsa verde, followed by the browned pork and its juices, and then the chicken stock; stir to combine, bring to a vigorous simmer, cover with a lid and place in the oven.
Cook the chili verde for 2 ½ to 3 hours, or until the pork is very tender, stirring occasionally.