Heat a large non-stick pan over a medium heat. Tip in the spinach and cook, stirring, until wilted (you won’t need any oil or water as it will steam in its own moisture). Leave to cool a little, then tip into a clean tea towel and squeeze out as much water as possible. Chop and put in a bowl.
Heat the knob of butter in a non-stick frying pan. Add the onion and cook for about 10 mins until softened. Tip into the bowl with the spinach, then stir in the feta using a fork until fully combined. Taste for seasoning.
Heat the oven to 190C/170C fan/gas 5. Brush a 24 x 4cm deep metal pie dish with some of the butter.
Lay a filo sheet on your work surface. Brush with some of the melted butter, then spread about 2 tbsp of the filling in a line along the longest edge of the pastry. Roll loosely into a long sausage (don’t roll it too tight as this might cause the pastry to split), then coil the pastry and put in the centre of the tin.
Repeat with the rest of the pastry and filling, arranging the coils around the central one. Brush the tops with more butter, then sprinkle with the nigella seeds. Bake for 25-30 mins until golden and crisp.