To make the crumble topping, tip the flour, custard powder and butter into a large bowl, then rub together using your fingertips until the mixture resembles coarse breadcrumbs. Stir in both sugars. The crumble topping can be prepared ahead and chilled for up to three days or frozen for up to three months. Roughly break up with your hands before using. If using straight from the fridge, the topping will be craggier; if you prefer it to be more loose, leave it to come back to room temperature before using.

Toss the chopped rhubarb, caster sugar, vanilla and cornflour together in a large bowl, ensuring the rhubarb pieces are evenly coated. Set aside to macerate for 30 mins. Tip the rhubarb into a large ceramic baking dish and spread into an even layer. Sprinkle over the crumble topping, making sure it covers the rhubarb completely. Can be assembled up to a day ahead and chilled until ready to bake, or frozen for up to three months. Defrost completely in the fridge overnight before continuing.

Heat the oven to 200C/180C fan/gas 6. Bake the crumble for 40-45 mins until the topping is golden and crisp and the rhubarb filling is bubbling at the edges. Leave to cool for 10 mins before serving with your choice of custard, cream or ice cream.