Step 1 Slice each steak horizontally through the middle to make 4 thin steaks. Mix Worcestershire sauce, mustard and garlic in a shallow non-metallic dish. Add steaks to sauce mixture and turn to coat. Cover dish; leave steaks to marinate for 30 minutes.
Step 2 Meanwhile, spray a large non-stick frying pan with oil and set over medium–low heat. Add onion; cook, stirring occasionally, for 3–4 minutes, or until onion is soft. Add capsicum and sugar to pan; cook, stirring occasionally, for 10 minutes, or until vegetables lightly caramelise. Transfer onion and capsicum to a bowl; set aside.
Step 3 Wipe frying pan clean, spray with oil and return to high heat. Drain steaks of excess marinade; cook for 2 minutes per side, or until done to your liking. Transfer steaks to a plate, cover loosely with a piece of foil and leave to rest for 2–3 minutes.
Step 4 Spread each slice of sourdough toast with 1 teaspoon of the extra mustard. Top with lettuce, tomato, cucumber, 1 slice of steak and caramelised onion and capsicum. Drizzle sandwiches with barbecue sauce and serve.