Heat the oven to 190C, and put the oil in a large saute pan on a medium-high heat. Once hot, add the onion, a teaspoon of salt and some pepper, and saute, stirring occasionally, for seven minutes, until the onion begins to colour.
Add the sumac, if using, and cook for a minute, then add the spinach in two batches, stirring continuously, to wilt, then cook for about seven minutes, until most of the liquid has evaporated. Transfer the mixture to a sieve, press to drain off any remaining liquid, then set aside to cool down a little.
Transfer the cooled spinach to a large bowl and add the herbs, feta, cheddar, pine nuts, egg and lemon zest. Fold gently to combine, then set aside.
Working one filo sheet at a time, and keeping the remaining sheets under a damp tea towel so they don’t dry out, cut each sheet lengthways into three strips, around 8cm wide x 35cm long. With the narrow side facing you, brush the exposed side of each strip with butter, then fold each strip in half by bringing the two narrow ends together, leaving you with 8cm x 17cm rectangles.
Brush the exposed side with butter again and, with the narrow end facing you, spoon about 25g-30g of the spinach mix along the bottom edge, keeping a few millimetres of pastry exposed around the edges. Gently roll the pastry upwards, encasing the filling inside a cigar-shaped parcel (don’t worry about encasing the exposed ends). This should make 24 cigars, about 8cm long x 3-4cm thick.
Arrange the cigars seam side down on two oven trays lined with baking paper, brush the tops with more melted butter, and bake for 22 minutes, or until browned all over.
Remove from the oven and leave to rest for at least 10 minutes before serving warm or at room temperature.
