Cook the rhubarb in a saucepan over medium-high heat with the sugar. The rhubarb will start to break down and release its own liquid.
Cook the rhubarb until it has softened and broken down, stirring frequently.
Taste the cooked rhubarb to see if you want to add more sugar.
Stir through the cornflour (cornstarch) to thicken the rhubarb mixture.
Take the saucepan off the heat.
Stir through the strawberries. The strawberries will start to soften from the heat of the cooked rhubarb.
Pour the cooked fruit into a large pie dish, about 23 cm or 9 inches wide.
Sprinkle over the raspberries.
Set the pie dish aside to cool slightly.
To make the crumble topping
Place the flour, salt and butter into a the bowl of a stand mixer with a flat-paddle attachment.
Beat on low speed until the butter starts to form large clumps with the flour, and the mixture resembles wet sand.
Alternatively, you can do this by hand by using your fingertips or a pastry blender to rub the butter into the flour, until you have a mixture which resembles wet sand.
Use a fork to stir through the sugars.
The crumble topping is ready to be used as is. However, if you like to have more crunchy bits in your crumble, you can use your fingertips to squeeze bits of the mixture together to form little clumps.
To assemble
Preheat the oven to 200°C/390°F (without fan).
Sprinkle the crumble topping over the rhubarb filling.
Bake for about 30 minutes or until the crumble is golden and the rhubarb filling is bubbling.
Kitchen Notes
QUANTITY OF FRUIT Please use the recipe for the rhubarb filling as a guide. You can add more or less of each fruit, depending on what you have. Sometimes I use less rhubarb, sometimes I use more strawberries, and sometimes I even leave out the raspberries.
MAKE AHEAD TIPS *You can make both the rhubarb filling and crumble topping ahead of time. * The rhubarb filling can be made up to several days in advance and kept covered in the fridge until you are ready to bake. * The crumble topping can be made in advance and kept in the freezer in zip-lock freezer bags. There is no need to defrost the crumble - simply use straight from frozen. * It is best to assemble the dish just before baking. If you cover the rhubarb filling with the crumble topping too far in advance, the crumble will absorb a lot of the liquid from the fruit, which will make the overall dish too dry.
OVEN & STOVE TEMPERATURES All recipes on this website have been tested on an induction stove and/or with a conventional oven (i.e. an oven without fan). All recipes on this website use temperatures for a conventional oven, unless otherwise mentioned. Convection ovens (i.e. fan-forced ovens) are typically 20°C/70°F hotter than conventional ovens, but please check your manufacturer’s handbook.