Put the chicken in a bowl. Massage in the olive oil, 2 tsp sumac, juice of half a lemon, garlic, thyme and chilli flakes, season. Leave to marinate for 30 mins.
Preheat the grill to high. Put the chicken on a baking tray and grill for 10-12 mins, until lightly charred and cooked through. Remove from the grill and set aside.
Meanwhile combine the yogurt, mint, remaining sumac and a squeeze of lemon juice in a bowl, season to taste. Set aside.
Spread each flatbread with a spoonful of the yogurt mixture, then top with lettuce and tomato. Divide the chicken between the flatbreads. Top with the pomegranate seeds, then drizzle over a little pomegranate molasses.