In a bowl, whisk together the olive oil, soy sauce, Worcestershire sauce, minced garlic, chopped rosemary, Dijon mustard, honey, and black pepper until well combined.
Marinate the Steak:
Place the cubed sirloin steak in a large resealable plastic bag or a shallow dish. Pour the marinade over the steak, ensuring all pieces are well coated. Seal or cover, and refrigerate for at least 2 hours, or overnight for more flavor.
Preheat the Grill:
Preheat your grill to medium-high heat. If using wooden skewers, soak them in water for at least 30 minutes before grilling to prevent burning.
Assemble the Kebabs:
Thread the marinated steak cubes onto skewers, alternating with pieces of bell pepper and red onion. Discard any leftover marinade.
Grill the Kebabs:
Grill the kebabs, turning occasionally, until the steak is cooked to your desired level of doneness and the vegetables are slightly charred, about 10-15 minutes for medium-rare.
Serve:
Remove the kebabs from the grill and let them rest for a few minutes. Garnish with fresh rosemary sprigs if desired. Serve hot.