Preheat grill to medium-high heat.
In a bowl, combine olive oil, lemon juice, sumac, salt, and pepper.
Thread beef cubes, bell pepper pieces, and red onion pieces onto skewers.
Brush the skewers with the sumac marinade.
Grill the skewers for about 3-4 minutes per side, or until the beef is cooked to your desired doneness and the vegetables are tender.
Serve hot and enjoy!