Cut the beef into evenly sized 1¼-inch cubes.
In a large bowl, whisk together the garlic, onion powder, soy sauce, Worcestershire sauce, balsamic vinegar, vegetable oil, and black pepper.
Add the beef to the marinade and toss until evenly coated. Cover and refrigerate for at least 3 hours or up to 24 hours.
Cut the bell peppers and onion into large pieces that match the size of the beef cubes.
Thread the marinated beef, bell peppers, onion, and mushrooms onto metal skewers, alternating the ingredients. Avoid packing them too tightly for even cooking.
Brush the assembled kebabs with the reserved marinade and lightly drizzle with olive oil.
Heat a grill or large skillet over high heat until hot.
Cook the Beef Kebabs for about 2 minutes per side, turning to cook all four sides, for a total of approximately 8 minutes or until the beef reaches your preferred doneness.
Baste occasionally with the reserved marinade while cooking.
Transfer the kebabs to a serving platter, cover loosely with foil, and let them rest for 3 minutes.
Garnish with chopped parsley and serve with your favorite dipping sauce.