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Heat the oven to 200C/180C fan/gas 6. Put the squash cut-side down in a roasting tin and roast for 45 mins-1 hr until completely soft and the skin has started to caramelise. Leave to cool in the tin.
Meanwhile, tip the onions into a large pan with a big pinch of salt and 400ml water and cook over a medium heat, stirring occasionally for around 25 mins until the onions are soft and the water has evaporated.
Wilt the spinach in two batches in a large, dry frying pan over a medium heat. Tip into a colander, leave to cool, then squeeze out as much water as you can. Tip the spinach into a bowl. Put the pan back over a medium heat and heat 1 tbsp oil, then fry the garlic until lightly golden. Tip this into the spinach and season to taste.
Drizzle the remaining oil into the pan with the onions and turn up the heat to medium-high. Cook, stirring regularly for 10-15 mins until the onions are golden and caramelised.
Scoop the soft squash flesh into a bowl and discard the skin and seeds. Mix in the ras el hanout, season, then leave to go cold along with the onions and spinach. All of the filling elements can be made up to a day ahead and kept covered in the fridge.
Roll the puff pastry out on a floured surface into a 50 x 30cm rectangle. Cut out two circles from the pastry, one 23cm and the other 28cm (you can use plates for templates), re-rolling the pastry after you’ve cut the smaller circle to fit the second one in. Transfer each circle to a sheet of baking parchment, put both on a baking tray and chill for 30 mins.
Heat the oven to 200C/180C fan/gas 6. Spread the squash mixture over the smaller circle of pastry, leaving a 2cm border around the edge. Pile on the pine nuts, caramelised onions and finally the spinach, shaping the filling into a dome as you go.
Brush the exposed pastry border with a little of the milk, then lay the second pastry circle on top, using your hands to snugly fit it around the filling to encase it. Crimp the edges with a fork to seal. Cut a small steam hole in the top of the pithivier, then lightly score the pastry using the back of a small knife or cocktail stick (as pictured). The pithivier can now be chilled for up to a day. Brush all over with the remaining milk, then bake for 30-35 mins until golden brown. Rest for 5 mins before serving.