Trim and score the flank steak: Pat the steak dry with paper towels and trim any large pockets of fat. Score lightly across the grain with shallow 1/8-inch cuts—this helps the marinade penetrate and prevents the meat from curling when searing.

Make the marinade: In a bowl whisk together 3 tbsp olive oil, 3 tbsp fresh lime juice, 2 tbsp soy sauce, 1 tbsp brown sugar, minced garlic, 2 tsp chili powder, 1 tsp smoked paprika, 1 tsp ground cumin, 1 tsp dried oregano, 1.5 tsp kosher salt, and 1 tsp black pepper. The oil carries fat-soluble flavors, the acid (lime) tenderizes and adds brightness, soy adds umami/salt balance, and the sugar helps caramelize the exterior during searing.

Marinate the steak: Place the steak in a shallow dish or zip-top bag and pour the marinade over it, massaging so the meat is evenly coated. Refrigerate for 30 minutes to 2 hours. (Why: brief marination infuses flavor without breaking down the meat too much—flank cooks best when not left for 24+ hours in acid.)

Prepare the vegetables and tortillas: While the steak marinates, slice the onion and bell peppers into uniform 1/2-inch strips so they cook evenly. Warm the tortillas: wrap stacked tortillas in foil and warm in a 300°F (150°C) oven for 10–12 minutes or briefly heat on a dry skillet. Warming prevents tearing and makes them more pliable for folding.

Preheat your pan or grill: Heat a heavy cast-iron skillet or a grill pan over high heat until very hot (it should shimmer and smoke faintly). A screaming-hot surface is essential to quickly sear the steak for a caramelized crust without overcooking the interior.

Sear the steak: Remove the steak from the marinade and let excess drip off (wipe away big globs so the pan doesn’t steam). Add 1 tbsp olive oil to the hot pan, then sear the steak 3–4 minutes per side for medium-rare (internal temp 130–135°F / 54–57°C). For a thicker piece, increase time slightly. Searing at high heat develops the Maillard crust—those browned bits are flavor gold.

Rest the steak: Transfer the steak to a cutting board and tent loosely with foil for 8–10 minutes. Resting lets juices redistribute; slicing too soon will cause them to escape and the steak to dry out.

Sauté the vegetables: While the steak rests, lower the heat to medium-high, add 1 tbsp butter to the same hot pan (use the beef fond left in the pan). Add the sliced onions and peppers, season with a pinch of salt and a sprinkle of the remaining chili powder and smoked paprika. Sauté, stirring occasionally, until edges are charred and vegetables are tender but still have bite, about 6–8 minutes. Cooking them in the steak juices and fond layers the flavors.

Slice the steak against the grain: Identify the grain (muscle fibers) and slice very thinly across it at a 45-degree angle. Thin, across-the-grain slicing shortens the muscle fibers and makes the beef seem more tender in each bite.

Finish and assemble: Toss the sliced steak back into the pan with the vegetables for 30–60 seconds to marry flavors and warm through. Squeeze a little fresh lime over everything, sprinkle chopped cilantro, and serve hot with warmed tortillas, lime wedges, and optional avocado or guacamole.

Serve and enjoy: Offer hot tortillas and let diners build fajitas with steak, peppers & onions, cilantro, a squeeze of lime, and optional avocado or salsa. Encourage quick eating while everything is still hot and juicy.