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Heat a large pan of water, add the potatoes, then cook for 8 mins until just soft. Add the peas 1 min before the end of cooking time. Drain, then set aside.
Meanwhile, heat the oil in a frying pan. Fry the onion until soft and slightly coloured. Add the curry paste and mustard seeds, then fry for a few mins more until the mixture smells fragrant. Carefully stir in the potatoes and peas, trying not to mash them. Add the lemon juice and coriander, then leave to cool.
Heat oven to 200C/fan 180C/gas 6. Unroll the pastry rectangle, place on a floured surface with the long side towards you and roll it out a little more to make it into a square shape. Cut into 4 squares, then cut each one in half so you have 8 long rectangles. Place 4 of them onto a baking sheet, brush the edges with beaten egg and place a quarter of the filling down the centre of each. Top with the remaining pastry, then pinch the edges together to make a sealed parcel. Brush with more egg, then bake for 20 mins until puffed and golden. If cooking from frozen, bake for 30 mins. Serve with fresh green salad and a spoonful of chutney.