Add the flank steak to a large pot and add enough water to just cover. Set over high heat and bring to a steady simmer. Reduce the heat to medium and use a spoon to discard any foam that rises to the surface of the water, around 10 minutes.
Add the carrots, onion, bell pepper, garlic, bay leaves, 1 teaspoon salt and ½ teaspoon black peppercorns. Bring the liquid to a boil, cover with a lid, reduce the heat to low and cook until the steak can be easily shredded, around 1.5 hours.
Transfer the steak to a tray and let cool. Discard the vegetables and pour the cooking broth through a sieve into a bowl. Reserve 1 cup of the cooking broth and set aside. Leftover broth can be stored in sealed jars and frozen up to 6 months.
Wipe the pot clean and return it to a medium heat. Add the avocado oil and heat until shimmering. Add the diced red pepper and diced onion. Season with a pinch of salt and cook, stirring occasionally, until the onions are soft and translucent, around 6 minutes. Add the garlic, cumin and oregano and cook, stirring, 60 seconds. Add the tomato paste and canned tomatoes and cook, stirring, until the liquid reduces by half in volume. Add the reserved beef broth and shredded flank steak and cook, stirring regularly, until the liquid reduces by at least half, around 10 minutes. Add the cilantro and stir to incorporate. Taste for seasoning and adjust with salt and pepper as desired.
Transfer the carne mechada to a large serving bowl and serve with the fried plantains and sliced avocado.