Step 1: Cut the beef into uniform 1 to 1.5-inch cubes. Place the beef cubes in a large bowl or a resealable bag.
Step 2: In a separate small bowl, whisk together the olive oil, lemon juice, minced garlic, grated onion, sumac, allspice, paprika, cumin, salt, and black pepper until well combined.
Step 3: Pour the marinade over the beef, ensuring all pieces are thoroughly coated. Cover and refrigerate for at least 4 hours, or preferably overnight (12-24 hours) for maximum flavor and tenderness.
Step 4: If using wooden skewers, soak them in water for at least 30 minutes before threading to prevent burning. If using metal skewers, no soaking is required.
Step 5: Thread the marinated beef cubes onto the skewers, leaving a small space between each piece to ensure even cooking.
Step 6: Preheat your grill to medium-high heat (400-450°F / 200-230°C). Alternatively, preheat your oven broiler or a cast-iron grill pan over medium-high heat.
Step 7: Lightly oil the grill grates or pan to prevent sticking.
Step 8: Place the beef kebabs on the hot surface and cook for 2-4 minutes per side, turning frequently, until browned with nice char marks and cooked to your desired doneness (e.g., 135-140°F / 57-60°C for medium).
Step 9: Remove the kebabs from the heat, transfer to a clean plate, cover loosely with foil, and let rest for 5-10 minutes before serving. This allows the juices to redistribute, keeping the beef tender and juicy.