Bring the flank steak to room temperature (20–30 minutes) and pat dry with paper towels. Allowing the steak to warm briefly promotes even cooking; patting dry removes surface moisture so the steak will sear and develop a flavorful crust rather than steam.

Make a simple rub: in a small bowl combine 1 tsp ground cumin, 1 tsp fine salt (or to taste), 1/2 tsp freshly ground black pepper, and 1 tbsp olive oil. Rub this over both sides of the steak. Rubbing spices into the meat coats the surface so the spice flavors toast and adhere during searing.

Prep the vegetables: core and slice the bell peppers into 8–10 mm (1/3–1/2 in) strips; halve, peel, and slice the onion into similar-width crescents. Mince the garlic. Uniform thickness ensures the peppers and onions sauté evenly so you don’t get undercooked strips.

Cut the lettuce into large leaves suitable for bowls or cups; wash, spin-dry, and chill briefly in the fridge while you cook. Chilled, crisp leaves make a pleasing contrast with warm steak and keep the final bowls bright and fresh.

Make the guacamole: halve and pit the avocados, scoop the flesh into a bowl, add the juice of 1 lime, 1/4 tsp salt, 1/4 tsp pepper, and the minced garlic. Mash to a chunky consistency with a fork — press some pieces through to create creaminess but leave chunks for texture. Lime brightens and prevents browning; a mix of smooth and chunky gives pleasing mouthfeel.

Heat a large cast-iron skillet (or heavy skillet) over medium-high heat for 3–4 minutes until very hot. A properly hot pan is essential for a quick, caramelizing sear on the steak and for getting color on the peppers and onion without overcooking.

Add 1 tbsp olive oil to the hot skillet, then add the steak. Sear undisturbed for 3–4 minutes to develop a deep brown crust, then flip and sear the other side 3–4 minutes. For a 500 g flank steak about 1–1.5 in thick, this typically reaches medium-rare to medium (internal temp 55–60°C / 130–140°F). Searing creates Maillard reaction flavors — brown, savory compounds that make the meat taste complex.

Remove the steak to a cutting board and tent loosely with foil to rest for 8–10 minutes. Resting lets the muscle fibers relax and reabsorb juices so the steak is moist when you slice it.

While the steak rests, return the same hot skillet to medium-high heat and add the remaining 1 tbsp olive oil. Add the sliced onions and peppers in a single layer and let them sit without stirring for 1–2 minutes to get some color, then stir/flip and continue to cook 5–7 minutes until softened and slightly charred. Season with a pinch of salt, 1/4 tsp ground cumin, and a grind of black pepper. Using the same pan captures the fond (browned bits) left from the steak, adding depth and a touch of steak flavor to the vegetables.

Thinly slice the rested steak against the grain into 4–6 mm (1/8–1/4 in) strips. Cutting against the grain shortens muscle fibers and makes each bite tender — especially important for flank steak, which is lean and fibrous.

Assemble the bowls: divide the prepared lettuce leaves among 4 plates to form a bowl or cup. Place a portion of sliced steak and a generous scoop of the pepper–onion mixture into each lettuce base. Top each with 1/4 of the guacamole. Squeeze extra lime if desired. Serving warm steak over chilled crisp lettuce creates a contrast of temperature and textures that feels intentional and restaurant-quality.

Finish and serve right away. If you like, offer extra lime wedges and a sprinkle of flaky sea salt on the steak for bright finishing accents. Quick service prevents the lettuce from wilting beneath the warm ingredients.