Prepare the flank steak marinade: In a medium bowl, whisk together the fresh lime juice (60ml or 4 tbsp), olive oil (60ml or 4 tbsp), chili powder (1 tbsp or 10g), cumin (1 tsp or 3g), smoked paprika (1 tsp or 3g), garlic powder (1 tsp or 3g), salt (1 tsp or 6g), and black pepper (0.5 tsp or 3g).

Marinate the steak: Place the flank steak in a large resealable plastic bag or shallow dish. Pour the marinade over the steak, ensuring it is evenly coated. Seal the bag or cover the dish and refrigerate for at least 30 minutes, or up to 4 hours for deeper flavor. If marinating for longer than 1 hour, flip the steak halfway through.

Prepare the vegetables: While the steak is marinating, prepare the vegetables. Wash the red, green, and yellow bell peppers. Remove the seeds and membranes. Slice each pepper lengthwise into thin strips, about 0.5cm (0.25 inch) thick. Peel the yellow onion and slice it thinly into half-moons, also about 0.5cm (0.25 inch) thick.

Preheat the grill: Preheat your outdoor grill or indoor grill pan to medium-high heat (approximately 200°C or 400°F). Ensure the grill grates are clean and lightly oiled to prevent sticking. If using a grill pan, add the vegetable oil (30ml or 2 tbsp) and let it heat up.

Grill the steak: Remove the flank steak from the marinade, letting any excess drip off. Discard the remaining marinade. Place the steak on the preheated grill. Grill for approximately 4-6 minutes per side for medium-rare, depending on the thickness of the steak. For medium, grill for 6-8 minutes per side. Use an instant-read thermometer to check for desired doneness: 57°C (135°F) for medium-rare, 63°C (145°F) for medium. The steak should have nice grill marks and a slightly firm feel.

Rest the steak: Once grilled to your desired doneness, transfer the steak to a clean cutting board. Tent it loosely with aluminum foil and let it rest for at least 10 minutes. This crucial step allows the juices to redistribute throughout the meat, ensuring a tender and flavorful steak.

Sauté the vegetables: While the steak is resting, add the sliced bell peppers and onions to the hot grill pan or directly onto the grill grates (if using a grill, you can place them in a grill basket or foil packet to prevent them from falling through). Sauté or grill the vegetables for 5-7 minutes, stirring occasionally, until they are tender-crisp and slightly charred. They should still have a slight bite to them and not be mushy.

Slice the steak: After resting, thinly slice the flank steak against the grain. Look for the direction of the muscle fibers and cut perpendicular to them. Aim for slices about 0.5cm (0.25 inch) thick. Slicing against the grain is essential for tender steak.

Warm the tortillas: While slicing the steak, warm the tortillas. You can do this by wrapping them in a damp paper towel and microwaving for 30-60 seconds, or by warming them briefly on a dry skillet or directly on the grill for a few seconds per side until pliable.

Assemble and serve: Arrange the sliced steak and sautéed vegetables on a platter or serve them directly from the pan (if using a grill pan). Serve immediately with the warm tortillas and your desired toppings like shredded lettuce, diced tomatoes, salsa, sour cream, guacamole, or shredded cheese.