Generously season the pork shoulder with salt, pepper, dried oregano, and cumin.
Place a layer of sliced onions on the bottom of the slow cooker. Add the seasoned pork shoulder. Top with additional onions. Add the orange quarters, lime and the can of beer.
Top with the lid and cook on low heat for 10 - 12 hours.
Once the pork is cooked, remove the bone, if there is one, also remove the orange and lime pieces from the slow cooker. Shred the pork with two forks.
Preheat the oven to 350° F.
This step is optional - but highly recommended. Place the pulled pork onto a baking sheet, reserving the juices in the slow cooker. Cook the pork for about 10 minutes, just until it's crispy on top. Place into a serving platter and pour the reserved juices back over the top.
Serve with charred corn or flour tortillas, avocados, queso fresco, fresh red onion, and a fresh mango salsa if desired.