Turn slow cooker to low. Peel and quarter onion. Peel garlic and smash with side of chef's knife. Add onion and smashed garlic cloves to slow cooker, along with orange juice, oregano, bay leaves, cinnamon. Stir.
Add pork tenderloin to cooker, spoon liquid over pork. Cover and cook 8 hours.
After 8 hours, remove pork from pot. Tent with foil and let the meat rest for 20 minutes.
Chop cabbage and cilantro. Char tortillas over stove top of gas range, or under broiler.
Shred the pork with two forks. Place in a skillet and warm it back up. Do not stir. Bottom pieces will get crispy, while top pieces stay tender.
Plate tacos with cabbage first, then pork, pickled onions, cojita, avocado, and cilantro. Serve with a slice of lime.