Combine olive oil, balsamic vinegar, minced garlic, Dijon mustard, oregano, thyme, black pepper, and salt in a large bowl. Whisk thoroughly until the marinade emulsifies and all seasonings dissolve completely into the liquid base.

Add cubed beef sirloin to the marinade, tossing gently to coat every surface with the flavorful mixture. Reserve 2 tablespoons of marinade separately for basting during cooking.

Let the beef marinate for 15 minutes at room temperature while you prepare your vegetables and soak your skewers. Thread the marinated beef onto skewers, alternating with red bell peppers, yellow bell peppers, red onion chunks, and whole mushrooms.

Preheat your grill to medium-high heat (around 400°F). Once the grill is hot and grates are clean, place your juicy grilled beef kebabs crowd recipe directly over the flames, spacing them 2 inches apart to prevent steaming.

Cook for 8-10 minutes total, rotating every 2 minutes for even charring and to ensure the beef reaches your desired doneness. Baste with reserved marinade during the final 2 minutes of cooking.

Remove from heat when beef reaches 135°F internal temperature for medium-rare doneness. Transfer to a serving platter and garnish with fresh rosemary while the beef kebabs rest for 3 minutes.