This is the base of my very favorite tacos. Added bonus is by cooking these all day in a slow cooker, your house smells amazing.
Note: Unless you're cooking for a dozen people, you're going to end up with a lot of leftover pork, but that's about the best problem you can possibly have.
GET GOING EARLY–in a perfect world, you're gonna give this thing at least eight hours on low.
Chop up your onion or shallot into small bits–you don't want big loops of onion, dice it.
Rub your hunk of pork generously with chili powder, oregano, pepper, salt, and a pinch or two of cumin. I tend to try and have a very light crust of this stuff on the outside of the pork.
Toss the hunk of pork, the onions, and the green chilis into the crock pot, set that sucker to LOW, and let it slow cook. I try to get mine in by 9am so I can eat around 6pm. If you're pressed for time, you can do this in about four hours on high, but the deliciousness factor is lowered.
Once the time is up, either shred the thing up in the crock pot or, if you can actually get it out without it falling apart, get it into a bowl and shred from there. That way, you're getting it out of what ends up being a GIGANTIC AMOUNT of juices. But again: oh no, your pork ended up sitting in a big vat of juice–we all weep for you.