In a small bowl, whisk together ¼ cup soy sauce, the 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 1 ½ 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 in a large bowl.
Pour marinade over the steak and veggies and stir to thoroughly coat the steak and veggies with the marinade.
Marinade for at least 30 minutes.
Thread steak and vegetables onto skewers.
Heat grill to medium-high heat.
Place kabobs onto hot grill and cook for 4-5 minutes per side.