Heat oven to 325°F. Season beef with kosher salt and pepper. Heat oil in a Dutch oven; brown beef in batches. Transfer to a plate.
Cook yellow onion 5 minutes. Add garlic, chili powder, cumin and oregano for 1 minute. Add broth, tomatoes and beef.
Cover; braise 2 1/2–3 hours until beef shreds easily. Shred into juices; simmer uncovered to reduce excess liquid if needed.
Meanwhile, toss red onion with 3 tablespoons lime juice and fine salt; stand 20 minutes. Stir remaining lime juice into beef.
Warm tortillas in a dry skillet. Fill with beef, avocado, drained onion and cilantro.