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Heat oven to 200C/180C fan/gas 6. Toss the squash in 1 tbsp oil and some seasoning and roast for 20 mins until soft. Meanwhile, fry the onion in the remaining oil for 10-15 mins over a low heat until golden and boil the potatoes for 8 mins until soft. Drain the potatoes and leave the veg to cool.
Reduce oven to 180C/160C fan/gas 4. Line a 20cm x 30cm baking tin with a long double thickness strip of parchment (this will help you to lift out the pie once cooked). Squeeze the two blocks of pastry together, then roll out on a floured surface – the pastry should be about the thickness of a £1 coin. Use the pastry to line the tin, leaving any excess pastry hanging over the edges.
Sprinkle the breadcrumbs over the base of the pastry. Layer in the beetroot slices, peppers, squash, onions, feta, potatoes, peas and herbs (in that order for a nice rainbow effect), seasoning well between each layer. Beat the eggs and pour slowly over the filling until most of it has sunk through the layers, saving a little to glaze the top. Brush some egg over the overhanging pastry then lay the rolled pastry sheet on top. Press the pastry sheets together, then use some kitchen scissors to trim the excess pastry, leaving about 2cm. Roll the edges in to seal, then press them firmly with a fork. Brush with egg, sprinkle with seeds, then use the fork prongs to mark out 12 portions. Bake for 1 hr 20 mins until the pastry is golden and crisp. Leave to cool completely before slicing, or chill for up to two days before serving.