Cut the flank steak into 3–4 inch chunks. Season well with salt and black pepper. In the pot (sauté mode if using an Instant Pot), sear the chunks on all sides until well browned, about 2–3 minutes per side. Remove and set aside.
In the same pot, sauté the onion, green pepper, and red pepper for 2–3 minutes until slightly softened. Add the garlic and cook until fragrant, about 1 minute.
Base Add the sofrito and tomato paste to the pot. Stir in the oregano, cumin, and smoked paprika. Cook everything together for 2–3 minutes, stirring frequently, until the paste deepens in color and the spices are fragrant.
Pour in the white wine and scrape up any browned bits from the bottom of the pot. Let most of the wine evaporate, about 2–3 minutes.
Add the tomato sauce, beef broth, and bay leaves to the pot. Stir to combine, then nestle the seared meat chunks back in so they’re submerged in the sauce. Seal and pressure cook on high for 55 minutes, then let pressure release naturally for 10 minutes before opening.
Transfer the meat to a cutting board and shred with two forks into long strands. Return the shredded beef to the pot and stir to coat in the sauce. Taste and adjust salt.
Stir in the pimientos and simmer uncovered for 10–15 minutes to thicken slightly. Remove the bay leaves. Let the ropa vieja rest in the sauce for at least 15 minutes before serving — this is what makes it extra flavorful.