Prepare the filling first. Melt the butter in a frying pan and fry the onion with a pinch of salt until soft, about seven minutes. Roughly chop the spinach and stir into the onion, along with the chilli flakes if you want.

Take off the heat, and crumble in the feta and stir in the yogurt and egg mixture. Check and adjust the seasoning and leave to cool.

Meanwhile (if cooking immediately) preheat the oven to 180C/350F/gas 4. Lay out the sheets. Filo comes in different sizes, so you may have to cut it to size. For each börek, you need a double layer of pastry in a rectangular shape approximately 25cm x 10cm. This can be done by folding a piece in half, or laying two sheets one on top of the other. Keep the sheets you’re not using yet under a damp kitchen cloth.

Grease a baking tray with butter. Use a pastry brush to paint the melted butter over the top surface of the pastry and between the two sheets so they stick together. Place a tablespoon of the cooled mix in the bottom left-hand corner of the rectangle in a triangular shape. Fold the lower left-hand corner over the mix to form a triangle. Fold the top left-hand side over and continue folding until the pastry is used up and you have a neat, tight triangle. Place on the baking tray and make the rest of the böreks. When finished, brush the tops with any remaining butter.

If cooking immediately, brush the egg wash lightly over the tops and sprinkle over the sesame seeds and nigella seeds. Place in the oven for 20-25 minutes until golden. Serve warm or at room temperature.

If freezing, skip the previous step. Place in single layers in a plastic container, with greaseproof paper between the layers. There is no need to defrost before cooking, simply egg-wash and scatter with seeds before cooking for 20-30 minutes.