1. To make the dough, mix the yeast, sugar, flour and salt in a large bowl. Make a well in the centre and pour in the yogurt, olive oil and 180-200ml lukewarm water; bring together to form a soft dough. Knead on a floured work surface for 8-10 minutes, until you have a smooth dough. Put into an oiled bowl, cover with oiled cling film and leave in a warm place until doubled in size – about 1 hour 30 minutes.
2. For the filling, heat the oil in a large frying pan over a medium-high heat. Add the salad onion and cook, stirring for 1 minute. Tip in the spinach; stir until wilted. Season and drain in a colander. Transfer to a bowl and allow to cool before stirring in the sumac and feta.
3. Divide the dough into 6 balls. On a floured work surface, roll out each ball as thinly as you can into a large circle (about 25cm diameter). Spread a thin layer of filling over one half, leaving a 2cm border, then fold over the other half and press the edges together to seal. Heat a large, non-stick frying pan over a medium heat. Brush one side of the flatbread with a little oil, then cook, oiled-side down, for 3 minutes, until blistered and charred in places. Brush the other side with more oil then flip over, lower the heat and cook for a further 3 minutes. Keep warm while you cook the remaining flatbreads, wiping the pan out each time with kitchen paper. Cut into wedges and serve warm.