Spray a baking sheet with cooking spray and place the tomatillos, quartered onion and garlic cloves on the baking tray. If using a jalapeño, add it to the baking tray with the other vegetables.
Roast the vegetables for 20 minutes flipping the vegetables at the 10 minute mark. The tomatillos should be lightly charred and the onion beginning to brown.
Transfer the roasted vegetables and roasted Hatch chiles (or jalapeño) and any remaining juice on the baking sheet to a food processor or blender. Blend until almost smooth. Add the cilantro leaves and kosher salt and pulse a few more times until mixed. Set aside or place in the refrigerator until .
Cut the pork roast into 1/2-inch by 1-inch pieces of pork. Season well with kosher salt and freshly ground black pepper.
Place a large skillet over medium high heat and add 1 tablespoon olive oil and the butter. Transfer half of the pork to the skillet and cook for about 4 minutes on each side, turning once or twice until the pork is browned on all sides. Transfer to a slow cooker and repeat with remaining pork.
Add all of the cooked pork and any bits and juice to the slow cooker. Pour all of the salsa verde on the pork and cover with lid. Set slow cooker to high and cook for 4-5 hours or low for 8-10 hours.
Serve warm with tortillas or use as sauce for enchiladas.
If you're looking to add more heat, add extra chopped chiles as a garnish.