Combine the cornflour, bilberries, lemon juice and sugar (apart from the 1tbsp) in a large mixing bowl. Cover and let stand for 15 minutes.
Preheat the oven to 190°C.
Roll the pastry out on a lightly floured surface to 1cm thickness. Line a 9 inch ceramic pie plate with the pastry, cutting off the overhanging pastry (of which there should be a lot). Gather the excess pastry into a ball and roll out to 1cm thickness roughly and reserve to one side.
Fill the pastry with the bilberry mixture. Cut the pastry into long strips that will act as decoration across the top of the pie. Arrange in a wide lattice pattern making sure each strip of pastry reaches the edge of the pie plate.
Seal well and brush the exposed parts of pastry with the beaten egg. Bake for 45-50 minutes until the pastry is cooked and the filing is glazed and hot.
Remove and allow to rest for a few minutes before serving with a small bowl of the thick cream on the side.
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