In a large bowl, combine the olive oil, minced garlic, chopped rosemary, salt, and black pepper. Add the steak cubes and toss to coat. Cover and marinate in the refrigerator for at least 30 minutes, or up to 2 hours for more flavor.
If using wooden skewers, soak them in water for 30 minutes to prevent burning. Preheat the grill to medium-high heat.
Thread the marinated steak, red bell pepper, green bell pepper, and onion onto the skewers, alternating between the ingredients.
Place the skewers on the grill and cook for 8-10 minutes, turning occasionally, until the steak is cooked to your desired doneness.
Remove from the grill and let rest for a few minutes before serving.