In a large mixing bowl, whisk together the olive oil, chopped fresh rosemary, minced garlic, soy sauce, Worcestershire sauce, fresh lemon juice, Dijon mustard, salt, and black pepper until well combined.

Add the cubed sirloin steak to the prepared marinade, ensuring all pieces are thoroughly coated. Cover the bowl and refrigerate for at least 30 minutes, or up to 4 hours for the most intense flavor. While the steak marinates, if using wooden skewers, immerse them in water to prevent burning on the grill.

While the steak is marinating, prepare your vegetables by cutting the bell peppers and red onion into uniform 1.5-inch pieces. If using cherry tomatoes, have them ready.

Preheat your grill to medium-high heat (around 400-450°F / 200-230°C). If you are using an oven, preheat it to 400°F (200°C) and line a baking sheet with foil for easier cleanup.

Thread the marinated steak cubes alternately with bell pepper, red onion, and cherry tomatoes (if using) onto the skewers. Try to leave a small space between each piece to ensure even cooking.

For grilling: Place the assembled kebabs on the preheated grill. Grill for 10-14 minutes, turning every 2-3 minutes, until the steak reaches your desired doneness (130-135°F for medium-rare, 135-140°F for medium).

For oven roasting: Place the kebabs on the prepared baking sheet. Roast for 15-20 minutes, flipping them halfway through, until the steak reaches your desired doneness. For a slightly charred finish, you can broil for the last 2-3 minutes, watching carefully to prevent burning.

Remove the cooked kebabs from the grill or oven and let them rest for 5 minutes before serving. This allows the juices to redistribute throughout the steak, resulting in a more tender and flavorful bite. Serve hot.