Prep and dry the steak: Pat the flank steak dry with paper towels. Removing surface moisture lets the meat sear rather than steam — that Maillard crust gives deep flavor and an attractive color. Trim any large pieces of silver skin but keep a little fat for flavor.

Make the dry rub and marinate: In a small bowl combine kosher salt, black pepper, chili powder and ground cumin. Rub the mixture evenly over both sides of the steak and let it sit at room temperature for 10–15 minutes (or up to 30 minutes in the fridge). Resting at room temp allows the seasonings to start penetrating and takes the chill off the meat so it sears more evenly.

Prep the vegetables and aromatics: Slice the red and green bell peppers and the onion into 1/2" strips so they cook quickly and match the bite size of the sliced steak. Mince the garlic and zest the lime. Cut the avocados in half, remove pits and scoop the flesh into a bowl for guacamole.

Make the quick guacamole: Mash the avocados with lime juice, a pinch of salt and 1 tbsp chopped cilantro (if using). Keep it slightly chunky — the texture contrasts nicely with the sliced steak. Acid (lime) brightens the avocado and prevents browning; salt enhances overall flavor.

Heat the pan: Place a large cast‑iron skillet or heavy stainless pan over high heat until smoking hot, about 3–5 minutes. A very hot pan is essential to quickly sear the steak and develop a crust without overcooking the interior.

Sear the steak: Add 1 tbsp olive oil to the hot pan and swirl to coat. Place the steak in the pan and leave it undisturbed for 2½–3 minutes to form a dark crust. Flip and sear the other side 2–3 minutes more for medium‑rare (internal temp ~130–135°F / 54–57°C). Thicker or thinner steaks will vary — use an instant‑read thermometer for accuracy. Searing locks in juices and creates savory browned compounds that taste 'meaty' and complex.

Rest the steak: Transfer the steak to a cutting board and tent loosely with foil for 6–8 minutes. Resting lets hot juices redistribute; slicing too soon causes the juices to run out and yields a drier steak.

Cook the peppers and onions: While the steak rests, return the same skillet to medium‑high heat. Add remaining 1 tbsp olive oil and the sliced peppers and onions. Cook, stirring occasionally, until they pick up brown edges but remain crisp‑tender, about 6–8 minutes. Add the minced garlic in the last 60 seconds so it softens and becomes fragrant but doesn’t burn. Browning adds sweetness and depth — don’t overcook or they’ll become mushy.

Slice the steak against the grain: Identify the muscle fibers (the grain) and slice the steak thinly across the grain into 1/4" slices. Slicing against the grain shortens the muscle fibers and makes each bite tender.

Warm the tortillas: Stack the tortillas and wrap in foil, then place in a 300°F (150°C) oven for 6–8 minutes, or heat each tortilla for 20–30 seconds per side in a dry skillet until pliable. Warm tortillas fold more easily and won’t crack when filled.

Assemble and finish: Toss the sliced steak back into the skillet with the peppers for a quick 30–60 second toss to marry flavors and reheat the beef. Squeeze a little lime zest and the remaining lime juice over the mixture for a bright finish. Taste and adjust salt.

Serve: Arrange tortillas on a platter. Spoon sliced steak and peppers onto tortillas, add a dollop of guacamole, a spoonful of salsa, and finish with chopped cilantro if using. Serve immediately so tortillas remain warm and fillings retain their texture.