Preheat your grill to medium-high heat.
In a large bowl, combine the olive oil, sumac, paprika, cumin, black pepper, cayenne pepper, minced garlic, lemon juice, and salt to create the marinade.
Add the lamb cubes to the marinade and toss to ensure they are well coated. Let the lamb marinate for at least 15 minutes.
While the lamb is marinating, prepare your vegetables. Skewer the tomato and red onion wedges onto separate skewers.
Once the lamb has marinated, thread the lamb cubes onto skewers, leaving a small space between each piece to ensure even cooking.
Place the lamb kebabs on the grill and cook for about 4 to 5 minutes on each side for medium-rare, or until they reach your desired level of doneness.
Grill the tomato and onion skewers alongside the lamb, turning occasionally until they are charred and tender, about 6 to 8 minutes.
Remove the kebabs from the grill and let them rest for a few minutes before serving.
Serve the lamb kebabs with the grilled tomatoes and onions, garnished with fresh parsley.