Season the flank steak with salt, garlic powder and pepper. Transfer to a bowl, cover and refrigerate anywhere from 1 hour to overnight.
In a cold stock pot or sauté pan, combine the flank steak, green pepper, halved onion, whole garlic cloves, bay leaves, lime juice, and peppercorns. Cover with water and place over high heat to bring to a boil. Drop the heat to medium-high to maintain a steady simmer and cook, adding more water if too much evaporates in the process, until the meat can be easily shredded with a fork, 1.5 to 2 hours.
Transfer the meat to a bowl and let cool. Strain the cooking liquid and reserve. Using your hands or two forks, shred the meat into thin strings. Set aside.
Preheat a cast-iron or stainless-steel skillet over medium-high heat. Add avocado oil and heat until shimmering. Add the shredded meat to the hot oil in a single layer and cook, undisturbed, until a crust forms, around 2 minutes. Add the onions and garlic, season with a pinch of salt and cook, stirring, until the onions are slightly soft and translucent, around 4 minutes. Add ¼ cup of the reserved stock and stir to coat (remaining stock can be frozen for up to 6 months). Taste the vaca frita for seasoning, adjust with salt as desired, and serve immediately.