Sprinkle half the spice mixture over the steak, coating both sides evenly. Set meat aside to come to room temperature, while slicing the capsicum and onion.
Preheat a heavy-based skillet over medium-high heat. Drizzle steak with a teaspoon of oil and season with a pinch of salt. Cook 3-4 mins each side. Remove from pan and rest 5 mins before thinly slicing.
In the same pan, drizzle in remaining oil and increase heat to high. Add onion and capsicum and cook 4–5 mins until soft and starting to char. Combine reserved spice mix, water, and lime juice in a small dish. Add to capsicum mixture and cook 1 min to thicken. Remove pan from heat, add sliced beef and toss to coat.
Serve beef and capsicum mixture with tortillas, avocado, tomato, lettuce, and lime. Let everyone help themselves to build their own perfect fajita!