Heat oven to 190C/fan 170C/gas 5. Place the pears in a medium-size pan, add the granulated sugar and cook on a medium heat until the fruit starts to soften and releases its juices, about 10 mins. Add the blackberries and bring back to the boil, then remove from the heat. Spoon the fruity mixture into 4 individual ovenproof ramekins, or 1 large baking dish.
Place the flour, butter and a pinch of salt in a large bowl and rub together with your fingers until the mixture resembles coarse breadcrumbs. Add the pistachios and demerara sugar, then stir to combine.
Sprinkle crumble evenly over the cooked fruit. This can be done up to 1 day ahead, or frozen for up to 1 month. Bake for 20-25 mins if small, 40 mins if large, until golden. If baking from frozen, add 15 mins cooking time. Remove from the oven, cool slightly, then serve with ice cream if you like.