Heat oven to 180C/160C fan/gas 4 and put a baking tray in the oven to preheat. On a lightly floured surface, roll out two-thirds of the pastry to a 5mm thick disc and use to line a 23cm pie tin, leaving any excess pastry to overhang. Spread the conserve onto the pastry base, then top with the blackberries and pears.
Roll out the remaining pastry on a lightly floured surface to 5mm thick. Using a fluted ravioli wheel, cut the dough into 12 x 23cm strips. To lattice the pastry, place a pastry strip horizontally on top of the pie, then lay a strip vertically, repeating to create a woven pattern. For an easier topping, simply arrange half the strips at 2cm intervals across the top of the pie. Repeat crosswise with the remaining strips. Seal and trim any excess pastry.
Place the crostata on the hot baking tray on the lowest shelf and bake for 40 mins. Set aside for 15 mins to cool then dust with icing sugar.