Preheat the oven to 375°F (190°C).

Blind bake the pie crust: roll out the pie crust and line a 9 inch pie plate with it. Line with parchment paper and fill with pie weights (dried beans, lentils, rice etc).

Bake for 20 minutes. Remove weights and parchment and bake for another 5-10 minutes, or until the bottom is golden brown and baked through. Let cool.

Make the filling: in a medium saucepan whisk together the sugar, cornstarch, salt and water. Add 2 cups (10 oz/284 g) of blackberries and bring to a simmer over medium-low heat, using a wooden spoon to slightly mash the berries as they cook.

Let the mixture simmer vigorously for 1 or 2 minutes, stirring constantly, until thickened and clear.

Remove from the heat, stir in the remaining 3 cups (15 oz/426 g) blackberries, butter and lemon juice until well combined and pour the filling into the baked pie crust.

Let set at room temperature for 3 hours before serving with freshly whipped cream.

Store, loosely covered, in the refrigerator for up to 2 days.