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Heat the oven to 200C/180C fan/gas 6. Toss the squash with half the oil and some seasoning in a roasting tin. Roast for 25-30 mins until just tender, tossing halfway through.
Meanwhile, heat the remaining oil in a non-stick frying pan and fry the leeks with a pinch of salt for 5 mins until beginning to soften. Stir through the pesto, beans and stock, then bring to a simmer and cook for 5 mins. Stir though the crème fraîche and squash, then spoon into a 26 x 18 x 5cm baking dish. Scrunch up the filo pastry and arrange on top of the filling. Brush with the egg and bake for 20 mins or until golden brown. Serve with the mixed salad leaves on the side, dressed with a little olive oil, if you like.