Zest and juice half the lemon (keep the other half for later). Peel and grate or crush the garlic. Add both to a large bowl with 2 tsp sumach and ½ tbsp oil. Season. Mix together. Peel and thickly slice the onions and add to the bowl. Tip in the chicken and turn to coat in the marinade. Set aside for 10 mins to marinate.
Spoon 2 tbsp of yogurt into a large bowl. Add 1 tbsp oil and 3 tbsp cold water. Whisk together with a pinch of salt. Tip in the flour and stir to make a soft, doughy mixture. Turn out onto your work surface and knead for 2-3 mins to form a smooth dough. Pop back in the bowl, cover with a tea towel and leave to rest.
Finely shred the lettuce. Halve the baby plum tomatoes. Roughly chop the parsley leaves.
Heat your grill to its highest setting. Line a baking tray with foil. Tip the sumach chicken, onions and baby plum tomatoes onto the tray. Grill for 8-10 mins, turning the chicken halfway through the cooking time, till it’s charred and cooked all the way through.
While the chicken cooks, mix the remaining yogurt with the tahini and a squeeze of juice from the remaining lemon half. Set to one side.
Divide the dough into 2 balls and roll each one out into a large circle. Brush 1 tsp oil onto both sides of the flatbreads. Heat a frying or griddle pan over a medium heat.
When the pan is hot, lay a flatbread in the pan and cook for 2 mins on each side till browned and puffed up. Slide the cooked flatbread onto a plate and cover with a tea towel to keep warm while you cook the other one.
Lay each flatbread on a plate. Top with the shredded lettuce and chopped parsley. Spoon over the hot sumach chicken, onions and tomatoes, drizzling over any juices from the tray. Drizzle over the tahini yogurt sauce and scatter over a pinch more sumach to serve.