1Pat the beef dry and season generously with salt and pepper.
2Heat the vegetable oil in a skillet over medium-high heat and sear the beef pieces on all sides until deeply browned, about 3 minutes per side. Transfer to the slow cooker.
3Lower the heat to medium, add the sliced onion to the skillet, and cook 4 minutes until softened. Stir in the garlic, chili powder, cumin, smoked paprika, and oregano for 30 seconds until fragrant.
4Add the chipotle peppers, adobo, diced tomatoes with their juice, and beef broth to the skillet and scrape up any browned bits. Pour everything over the beef in the slow cooker.
5Cover and cook on low for 8 hours, until the beef falls apart easily.
6Transfer the beef to a cutting board, shred with two forks, and discard any large pieces of fat. Skim excess fat from the cooking liquid, then return the shredded beef to the pot with the lime juice and enough liquid to keep it moist. Season with salt to taste.
7Warm the tortillas, fill with the beef, and top with cilantro, cotija, and lime wedges.