In a small bowl or glass measuring cup, whisk together the balsamic vinegar, soy sauce, Worcestershire sauce, 2 tablespoons olive oil, chopped rosemary, minced garlic, Dijon mustard, black pepper, and onion powder until combined.
Trim and cut the top sirloin into 1-inch pieces and place them in a large zip-top bag.
Pour the marinade over the steak in the bag, seal, squeeze out excess air, and refrigerate for at least 1 hour or up to 8 hours.
When ready to grill, remove the steak from the marinade and discard any excess marinade.
Preheat the grill to medium-high heat.
Toss the mushrooms, bell peppers, and red onion with 1 to 2 tablespoons olive oil and season lightly with salt and pepper.
Thread the steak and vegetables onto skewers, alternating steak and vegetables.
Place the kabobs on the hot grill and cook 3 to 5 minutes per side, turning as needed so all sides are evenly browned and steak reaches desired doneness.
Remove the kabobs from the grill, sprinkle with chopped fresh parsley if using, and serve immediately.