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Heat oven to 220C/200C fan/gas 7. Put the squash, onions and chilli flakes in an ovenproof pie dish (or 4 individual dishes). Season and cook for 20 mins until the squash is tender and the onions are starting to char at the edges.
Meanwhile, put the spinach in a colander and pour over a kettleful of boiling water. Squeeze out any excess liquid and stir into the squash mix. Dot over the feta, crumple up the pastry and place on top, then brush with the oil. Return to the oven and cook for a further 15 mins until the pastry is golden and crisp. Serve with a green salad, if you like.