Preheat your oven to 350°F (175°C).
In a medium bowl, mix together the flour and half of the sugar. Add the cold cubed butter and blend with a pastry cutter or your fingers until the mixture resembles coarse crumbs.
In another bowl, combine the remaining sugar, lemon zest, orange zest, orange juice, lemon juice, and cornstarch. Mix until well blended.
Pour the citrus mixture into a baking dish. Evenly sprinkle the crumble mixture over the top.
Bake for 30 minutes or until the crumble topping is golden brown and the citrus filling is bubbly.
Allow to cool slightly before serving. Enjoy your delightful Citrus Crumble!