This easy rhubarb crumble recipe is made from simple ingredients you probably have in your pantry! This rhubarb crisp is baked in stages to ensure the filling sets properly and the crumble topping bakes without burning.
Preheat the oven to 400 ºF (200 °C). Place a 8x8x2 inch (20x20x5 cm) ceramic baking dish (like a deep square Pyrex or a ceramic baking dish with at least 2 inch sides) on a parchment-lined sheet pan. Set aside.
Slice washed rhubarb into 1 inch (2.5 cm) pieces. Place half the rhubarb in the baking dish. Sprinkle the sugar, cornstarch, and water over top, then toss the ingredients together to evenly coat all the rhubarb. You can use your hands for this.
Cover the baking dish with a sheet of aluminum foil then bake the rhubarb for 30 minutes. When you carefully peel back the foil, you will see the rhubarb should be soft and has released a lot of water.
Fold in the rest of the rhubarb and the vanilla bean paste (or extract) to make sure the filling is well mixed using a heatproof spatula. You can also add a few raspberries for colour. Set aside.
Crumble topping
Drop the oven temperature to 375 ºF (190 °C).
In a large bowl, whisk the flour, sugar, and salt.
Drop the cubes of butter into the bowl and using your hands toss them in the flour mixture to coat them. Work the butter into the flour using your finger tips to form a crumbly mixture.
Sprinkle the crumble over the rhubarb filling. Bake the rhubarb crumble, uncovered, until the filling is bubbling and very hot, even in the middle, and the topping is golden. This will take about an hour, if not more.
Let cool for at least an hour (if not several hours) so that the juices set. Serve plain or topped with vanilla ice cream.