Follow these simple steps to create Barbacoa Tacos with Pickled Red Onions:

1. Marinate the Meat: In a bowl, combine the cumin, oregano, chili powder, garlic powder, onion powder, salt, and pepper. Rub the spice mixture all over the beef chuck roast and let it marinate for at least 2 hours (preferably overnight in the fridge).

2. Prepare the Slow Cooker: Place the marinated meat into a slow cooker. Add the beef broth, sliced onion, apple cider vinegar, and bay leaf.

3. Cook: Cover the slow cooker and cook on low for 6 to 8 hours or on high for 3 to 4 hours, until the meat is fork-tender.

4. Shred the Meat: After cooking, remove the beef from the slow cooker and place it on a cutting board. Use two forks to shred the meat, then return it to the slow cooker to soak up the juices.

5. Make the Pickled Onions: In a saucepan, combine water, apple cider vinegar, sugar, and salt. Bring to a boil, stirring to dissolve the sugar and salt. Pour the hot mixture over the sliced red onions in a jar or bowl. Let sit for at least 30 minutes or refrigerate overnight for a stronger flavor.

6. Warm the Tortillas: Heat the corn or flour tortillas in a skillet for about 30 seconds on each side until warm and pliable.

7. Assemble the Tacos: Fill each tortilla with shredded barbacoa and top with pickled red onions. Sprinkle with chopped cilantro and add slices of avocado.

8. Serve: Squeeze fresh lime juice over the tacos for an added zing, and enjoy!