In a small bowl, whisk together ¼ cup soy sauce, juice from 1 lemon, ¼ cup olive oil, ¼ cup Worcestershire sauce, 2 teaspoons Italian seasoning, 1 teaspoon black pepper, ½ teaspoon salt, and ½ teaspoon garlic powder.
Place 1½ pounds beef sirloin cut into 1-inch pieces, 1 large red onion cut into 1-inch pieces, 1 red bell pepper cut into 1-inch pieces, and 1 yellow bell pepper cut into 1-inch pieces into a large bowl.
Pour the marinade over the steak and vegetables and stir well to coat evenly.
Cover and marinate for at least 30 minutes.
Thread the marinated steak and vegetables onto skewers.
Preheat the grill to medium-high heat.
Place the kabobs on the grill and cook for 4–5 minutes per side, turning occasionally, until the steak is cooked to your desired doneness and the vegetables are tender.
Remove from the grill and serve immediately. Enjoy!