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.
Place 800 grams of the cherries into a large saucepan, along with the sugar, salt, vanilla and the lemon zest & juice. Set the pan over a medium-low heat & cook gently until the sugar has dissolved & the fruit is soft. This will take 5-10 minutes. Make sure to stir the fruit regularly! Increase the heat to medium & bring the fruit to a simmer. In the meantime, place the cornflour into a small bowl & mix in 2 tbsp of cold water, to form a thin, smooth paste.
Next, stir the cornflour into the fruit & cook out for a minute or two, until thick. Take the pan off the heat & add in the remaining cherries & the butter then stir, to melt & combine.
Transfer the filling into the greased dish then sprinkle the chilled crumble topping over the top, so that the cherries are 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.