In a small bowl, combine aleppo pepper, sumac, cumin, salt and pepper; whisk to blend and set aside.
In a large bowl, add lamb and onion; mix well. Add spice blend and mint; blended all the ingredients before forming 4 equal sized balls. Roll each ball into a cylinder shape of about 8-inches long. Gently slide a skewer through the middle; tighten meat around the skewer. Place on a baking sheet while doing the others. Transfer to the refrigerator for 2 hours.
Meanwhile make the sauce by combining all the ingredients. Whisk well, cover and chill until ready to serve.
About 15 minutes before taking the kebabs out of the fridge, soak onion slices in very cold water for 12 to 15 minutes to remove the bite from the raw onions. Next thread tomatoes on a skewer and take naan bread out ready for the barbecue.
Brush grates with oil and fire up the grill. When it reaches 450-500ºF, place kebabs, rotating every couple minutes, until evenly browned and slightly charred; cook for about 7-8 minutes. Remove from the BBQ and let them rest while grilling the other food.
Place flatbread and tomatoes on the barbecue, grill for 1 ½ to 2 minutes before flipping the bread and rotating the tomato skewer; cook for another 1 ½ to 2 minutes. Remove the naan bread but leave the tomatoes for a couple more minutes. Serve with sliced red onion, parsley and yogurt sauce. Makes 4 kebabs.