In a large skillet, heat the oil over medium-high heat. Begin to brown the beef ribs in batches until browned on both sides, about 3 to 4 minutes per side. Place them inside a large slow cooker once they've browned.
Make the sauce:
In a blender, combine the chipotle peppers in adobo sauce, garlic, tomato paste, lime juice, apple cider vinegar, cumin, salt, and beef stock. Blend until the sauce is homogenous. Transfer to the slow cooker and pour over the beef. Add half of the chopped onion, reserving the remaining half for serving, and the bay leaves.
Cook the meat:
Cover and cook for 6 hours on high or 10 hours on low, until the meat is falling off the bone.
Remove the ribs from the sauce. Remove any excess fat from the ribs; discard the bones and shred the meat. Using a slarge spoon, skim the fat from the surface of the sauce in the slow cooker. Transfer the meat back to the slow cooker and combine with the sauce.
Assemble the tacos:
Serve the beef inside a tortilla and top with the reserved chopped onions, sliced avocados, pico de gallo, and a dab of sour cream.