Add the steak to a resealable bag or container with the salt, cumin, Aleppo pepper, sumac, coriander and olive oil. Grate the onion and add both the onion and its juices. Massage everything together and refrigerate for at least 2 hours and up to 6 hours.
Heat a grill for medium-high direct cooking, around 400°F.
Thread the steak tightly onto metal skewers, about 8–10 pieces per skewer. Grill for 4–5 minutes, flip, then cook another 4–5 minutes, or until nicely charred and cooked to your liking. Remove and cover loosely with foil.
Combine the sumac onions ingredients in a bowl and toss well with your hands, separating the onion strands. Set aside to marinate while you finish everything else.
Combine all yogurt flatbreads ingredients in a bowl and mix until a dough forms. Cover and rest for 15 minutes.
Divide the dough into 8 pieces and roll each piece to about ¼-inch thick. Brush both sides with olive oil and grill for 2–3 minutes per side.
Serve the kebabs with the sumac onions and warm yogurt flatbreads.