Pour the olive oil into a large frying pan on a low heat. When hot, add the onions and cook for 25 mins until soft and translucent. Stir the chard stalks into the pan, cover with a lid and cook for another 15 mins until softened. Add half the chard leaves, stir until wilted, then add the rest. Cover with a lid and steam for 5 mins until completely wilted. Remove the lid, add 1 tsp salt along with a good grinding of pepper. Stir well, increase the heat to medium and cook for 5-8 mins until the remaining liquid has evaporated. Add the chillies, garlic, sumac and feta, mix well, then leave to cool for 10 mins.

Meanwhile, make the dough. Put 430ml warm water in a large bowl with the vinegar, oil and 2 tsp sea salt. Mix well and tip in the flour. Bring together into a ball using your hands and turn out onto a lightly floured work surface. Knead until the dough is smooth and elastic, about 10 mins. Put the dough back in the bowl, cover with a damp tea towel and leave it to rest for at least 30 mins. You will end up with around 1.3kg of dough.

Once rested, divide the dough into eight balls, roughly 160g each. Lightly dust a work surface with a little more flour and roll the dough balls into thin 35cm wide circles. Brush the dough generously with 1 tbsp of the olive oil and butter mixture. Fold over two sides of the dough so the edges meet at the centre. Spoon an eighth of the filling onto the centre, leaving a 1cm gap around the edges. Fold in the other two halves of the dough over the filling and press all the edges together to seal and form a rectangle. Place on a large baking tray and cover with a tea towel. Repeat until all the filling and dough has been used up.

Heat a large frying pan over a low heat. When hot, brush with some of the remaining olive oil and butter mixture and add a filled gözleme. Cook for 5-7 mins until golden and crispy. Brush the top with more of the olive oil and butter mixture and use a spatula to flip over and cook for another 5-7 mins. Turn up the heat to medium and fry for 2 more mins on each side or until golden. Set aside on a large platter and cover with a tea towel. Continue until all the gözleme have been cooked. Serve hot.