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1
Process flour, polenta and butter in food processor until mixture resembles breadcrumbs. Combine egg and 1/4 cup ice-cold water; reserve 1 tbsp for brushing. Add remaining egg mixture to flour mixture, process until pastry comes together. Turn onto work surface; knead lightly into a ball. Wrap in plastic wrap; refrigerate for 2 hours.
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2
Meanwhile, for the filling, heat 1 tbsp of the oil in a large skillet on medium heat. Cook chicken, in batches, until browned. Drain on paper towels. Heat remaining 1 tbsp oil in same pan. Cook leek and sweet potato, stirring, for 3 mins. Add wine and stock; cook, stirring occasionally, for 10 mins, or until most of liquid has evaporated. Remove from heat; stir in chicken, combined creme fraiche and flour, then tarragon. Cool.
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3
Cut pastry into 2 pieces, 1 slightly larger than the other. Roll out larger piece on a floured work surface until large enough to line a deep 9-inch springform pan, about a 13-inch round. Press pastry into bottom and sides of pan. Cover; refrigerate for 1 hour.
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4
Preheat the oven to 400u00b0F. Place a baking pan in oven to preheat. Spoon chicken mixture into crust; brush edges with a little reserved egg mixture.
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5
Roll out remaining pastry on floured work surface until large enough to cover filling. Lift pastry onto filling; press pastry together to join. Trim edge, then press to seal with a fork. Brush top of pie with remaining reserved egg mixture; sprinkle with additional polenta. Place pie on hot pan.
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6
Bake for 20 mins. Reduce oven temperature to 350u00b0F. Bake for a further 25 mins or until pastry is golden. Let pie stand in pan for 10 mins before serving.
