Caramelize the onions: In a large skillet, heat the vegetable oil over medium heat. Add the sliced onions and sprinkle with about 1/2 teaspoon of the salt. Cook, stirring occasionally, until the onions are soft, deep golden, and caramelized, 20 to 25 minutes. If they start to brown too quickly, turn the heat down and add a splash of water to keep them from burning.
Make the 1 mixture: When the onions are caramelized, turn the heat to low. Pour in the dark beer and beef broth, scraping up any browned bits from the bottom of the pan. Stir in the brown sugar, Worcestershire sauce, garlic powder, the remaining salt, and about 1/2 teaspoon of the black pepper. Let this come to a gentle simmer for 2 to 3 minutes, stirring to dissolve the sugar. This is your 1 rich mixture for the slow cooker.
Place the pork in the slow cooker: Set the raw pork butt into the bottom of a large slow cooker (5- to 7-quart size). Sprinkle the remaining black pepper over the pork.
Pour the 1 mixture over the pork: Carefully pour the hot beer-and-onion mixture evenly over the raw pork butt in the slow cooker, making sure the caramelized onions are scattered all around and on top of the meat. The pork should be mostly surrounded by liquid and onions.
Slow cook until tender: Cover the slow cooker with the lid. Cook on LOW for 8 to 10 hours, or on HIGH for 4 to 5 hours, until the pork is very tender and easily pulls apart with a fork.
Shred and skim: Transfer the cooked pork butt to a large bowl or cutting board. Let it rest for about 10 minutes, then use two forks to shred the meat, discarding any large pieces of fat. Skim excess fat from the surface of the cooking liquid in the slow cooker with a spoon.
Combine and season: Return the shredded pork to the slow cooker, stirring it into the beer-onion juices so the meat soaks up the flavor. Taste and adjust seasoning with additional salt and pepper if needed. Let it sit on WARM for 15 to 20 minutes to marry the flavors before serving.
Serve: Spoon the pork and plenty of the onions and juices over mashed potatoes, noodles, or rice, or pile onto sandwich rolls. Store leftovers in the refrigerator for up to 4 days or freeze for up to 3 months.