Start the pickled onions: Combine white wine vinegar, sugar, and 1 teaspoon salt in a small saucepan and heat until dissolved. Place sliced red onion in a jar, pour the warm liquid over, and refrigerate for at least 1 hour.

Season the beef chuck roast generously with cumin, oregano, smoked paprika, salt, and black pepper, rubbing the spices into all sides.

Place the seasoned beef in a slow cooker. Add the quartered white onion, minced garlic, chipotle peppers, adobo sauce, beef stock, apple cider vinegar, and bay leaves.

Cover and cook on LOW for 4 hours or until the beef is fork-tender and reaches an internal temperature of at least 63°C (145°F). For optimal tenderness, continue cooking until it shreds easily.

Remove the beef from the slow cooker and shred using two forks. Discard the bay leaves and onion quarters. Return the shredded beef to the cooking liquid and stir in lime juice. Keep warm.

Prepare the avocado topping: Halve the avocados, remove the pits, and slice or mash the flesh. Toss with fresh lime juice and salt to prevent browning.

To assemble, place tostada shells on serving plates. Spoon generous portions of warm barbacoa onto each tostada.

Top with sliced or mashed avocado and a heap of drained pickled red onions.

Garnish with fresh cilantro leaves and serve immediately with lime wedges on the side.