I find the flavor is best when the pork is seared first. My slow cooker sears the meat, but if yours does not, heat a large pot over medium to high heat and add the oil. Sprinkle the pork roast with the salt and pepper, then add it to the pot or slow cooker and sear on all sides until deeply golden brown. If necessary, transfer it to the slow cooker.
At this point, you want to roast or broil the tomatillos, onions and peppers. I have a gas stovetop so I roast them over an open flame! I roast the tomatillos, the onions, the chilis and peppers until they are a charred. I throw them all in a bowl and let them cool slightly. Peel the peppers and chilies and remove the seeds (I do!). Add the tomatillos, onions, chilies and peppers to the blender. Add in the garlic and the cilantro. Blend until the mixture is pureed.
Pour the mixture over the pork in the slow cooker and cook for 8 hours over low heat.
After 8 hours, the pork will easily shred with kitchen tongs or forks. My favorite way to serve this is either over rice or in charred tortillas as tacos. Add on some fresh cilantro, avocado, sliced jalapeños and lime spritzes.