Preheat an oven to 180°c/160°c fan (356°f/320°f).Grease the inside of a round 10" ovenproof dish with butter then set aside. Add the lemon juice into a large saucepan then set to one side.
Peel, quarter & core the apples & pears then cut into 1-2cm thick pieces. As you cut the fruit, place the pieces into the pan & mix with the lemon juice, to stop them going brown.
Next, add the sugar, butter, vanilla, & salt to the fruit & stir to coat.
Set the pan over a medium-low heat then cook until the fruit has started to soften, stirring regularly. This will take 10 minutes or so.
Next, add the blackberries into the apples & pears then cook for another minute or two, stirring frequently until the fruit has turned red.
Place the cornflour into a bowl & stir in enough cold water to make a thick paste. Add this to the fruit then cook out for another 1-2 minutes, until the juice has thickened.You'll need around 1 tbsp of cold water. Transfer the filling into the greased dish then sprinkle the chilled crumble topping over the top, so that the fruit is completely covered.
Bake the crumble in the preheated oven for 25-30 minutes, until the topping is crisp & golden and the filling is bubbling.I'd recommend cooking the crumble on a baking tray, just in case any juices bubble over! Once baked, spoon the crumble into bowls then serve with scoops of vanilla ice cream or a generous amount of hot vanilla custard.