Mix the spice rub. Combine 2 tsp Kosher salt, 2 tsp cumin, 1 tsp chili powder, 1 tsp oregano, 1 tsp garlic powder, ½ tsp onion powder, and ¼ tsp black pepper in a small bowl, then whisk well. Season the pork shoulder pieces on all sides.
Sear the pork. Heat a large pot over high heat, then add 2 Tbsp avocado oil. When very hot, add half of the pork, searing until golden brown on all sides, about 6-8 minutes total per batch. Transfer to a large plate, then repeat with remaining pork and set aside. Do not drain the drippings at the bottom of the pan!
Sauté the onion. Add thinly sliced onion to the hot pan, then scrape up all the brown bits. This equals flavor! Cook until softened, about 3-5 minutes.
Braise the pork. Return the seared pork to the pot, then add ½ cup chicken stock or beer (sometimes I'll add both). Add the zest and juice of 1 orange and 2 limes. Bring to a boil, then reduce the temperature to a simmer. Cook for 3-3½ hours, or until the pork is fall apart tender. Keep an eye on the liquid! If all the liquid cooks off, everything at the bottom of the dutch oven will burn. You shouldn't need to add any additional stock, beer, or water, but check it every 45 minutes or so.
Cool slightly. Transfer the pork carefully to a large mixing bowl or tray and allow to cool about 20 minutes, until you're able to handle the meat. Pull or shred the pork. It should press apart easily in your hands.
Crisp the carnitas! You can either serve the carnitas immediately after shredding, return to the pot along with the drippings, or transfer to a parchment-lined baking sheet and broil for crispy edges. To do this, place under a high for up to 5 minutes — just keep an eye on it so it doesn't burn.
Enjoy! Serve the pork in tacos that have been warmed with chopped onion, fresh cilantro, and lime wedges.