Pat the beef steak dry with paper towels and cut into 3-inch chunks. Season generously with salt and black pepper on all sides.

In a small bowl, combine cumin, oregano, chili powder, paprika, and cayenne pepper to create a spice rub. Coat the beef chunks thoroughly with this mixture.

Heat olive oil in a large Dutch oven over medium-high heat. Sear the beef chunks in batches until browned on all sides, about 3-4 minutes per side. Remove and set aside.

In the same pot, add diced onions and cook until softened, about 5 minutes. Add minced garlic and cook for 1 minute until fragrant.

Return the beef to the pot. Add bay leaves, lime juice, and enough water to cover the meat halfway. Bring to a boil, then reduce heat to low, cover, and simmer for 2.5-3 hours until meat is fork-tender.

While the beef cooks, prepare the salsa verde: Heat a dry cast-iron skillet over high heat. Add whole tomatoes and jalapeño peppers (if using), charring them on all sides until blackened in spots, about 8-10 minutes total.

Transfer charred vegetables to a blender. Add fresh cilantro, lime juice, minced garlic, salt, and a splash of water. Blend until smooth but still slightly chunky. Adjust seasoning to taste.

Once beef is tender, remove from pot and shred using two forks. Discard bay leaves. Return shredded meat to the cooking liquid and toss to coat.

Warm tortillas in a dry skillet or over an open flame until pliable and slightly charred.

Assemble tacos by placing shredded barbacoa in warm tortillas, topping with charred salsa verde, diced onions, fresh cilantro, and a squeeze of lime juice. Serve immediately.