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For the filling, wash the nettles well but don’t drain them too thoroughly. Heat the solid butter in a large frying pan. When it’s sizzling and has turned a nutty brown, add the nettles and cook for about 6 mins until wilted. Leave to cool.
Using a tea towel, squeeze as much liquid out of the nettles as you can, then roughly chop and tip into a bowl. Mix in the feta, parmesan, lemon zest and juice, about two-thirds of the egg and some nutmeg, and season generously. The filling should be loose but not sloppy.
Heat oven to 200C/180C fan/gas 6. To assemble the spanakopita, lay three sheets of filo end-to-end on your work surface, overlapping by about 5cm. Brush each piece with melted butter and top with three more sheets, then brush with more butter and add the final sheet to the middle for extra support. Spoon the nettle mix along the edge, about 2cm wide, and tuck over the short ends to stop any filling from coming out. Roll the pastry into a long sausage shape. Starting with one end, roll up the sausage into a spiral shape about 20cm wide. Put the spanakopita in a shallow round pan, like an ovenproof frying pan, if it fits. If not, slide it onto a buttered baking tray.
Brush the pie liberally with the remaining egg and scatter with the sesame seeds. Bake for 40-45 mins or until golden brown. Leave to cool until just warm, then serve in slices with a peppery salad.