Preheat the oven to 180°C.
Combine the cornflour, strawberries and sugar (apart from the 1 tbsp) in a large mixing bowl. Cover and let stand for 15 minutes.
Meanwhile, 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 a 9 inch circle of pastry and reserve to one side.
Fill the pastry with the strawberry mixture and dot the surface with the butter. Cover with the reserved pastry and seal well at the edges. Brush with the milk and sprinkle over the remaining 1 tbsp of caster sugar. Make three slits in the centre of the pastry to release steam when baking.
Bake on a baking sheet for 50-55 minutes until golden brown. Remove when ready and allow to stand for 5 minutes before serving. Serve with the fresh strawberries in a pot on the side.
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