In a mixing bowl, combine the flour and sugar for the crumble topping. Add the cold butter and rub it into the flour mixture until it resembles breadcrumbs.
In another bowl, mix the chopped rhubarb, sugar, and lemon juice for the filling. Transfer this mixture to a baking dish.
Sprinkle the crumble topping evenly over the rhubarb mixture.
Bake in the preheated oven for 40 minutes or until the topping is golden brown and the filling is bubbling.