Sear the beef

• Season the flank steak generously with salt and black pepper.
• Heat oil in a Dutch oven over high heat and sear the steak on both sides until browned.
• Set the steak aside.

Build the braising liquid

• Lower the heat and cook the chopped onion and garlic until softened.
• Add the cumin, oregano, ground cloves, paprika and red wine vinegar, stirring to coat.
• Add the canned tomatoes, broth, bay leaves and cinnamon stick, then return the beef to the pot.

Braise the beef

• Cover and simmer gently until the beef is fork tender.
• Shred the beef with two forks and return it to the sauce.

Pickle the onions

• Bring the white vinegar, water, sugar, salt and black peppercorns to a boil.
• Pour the hot brine over the sliced red onion and smashed garlic in a bowl.
• Let cool and refrigerate until needed.

Make the avocado crema

• Blend the avocados, sour cream, garlic, lime juice, cilantro, jalapeño and salt until smooth.
• Adjust the seasoning to taste.

Assemble the tacos

• Warm the corn tortillas in a pan.
• Fill with shredded beef, top with melting cheese, chopped cilantro, onion and pickled onions.
• Drizzle with avocado crema and serve.