Preheat oven to 200°C/375°F (180°C fan).

Filling: Place rhubarb, apple, sugar and orange zest in a large mixing bowl and toss well. Spread evenly in a 30 x 20cm baking dish (12 x 8") (Note 3)

Crumble topping: Place topping ingredients in a bowl and mix with a wooden spoon until all the flour is incorporated (use cutting motion/ smearing up wall, as needed). It will be a sandy mixture.

Scrunch & top: Grab handfuls of crumble and enclose your fist to press into lumps. Break into clumps and scatter over the filling. (Note 4 explains this unusual step!).

Bake for 35 minutes or until rhubarb is soft and the crumble is golden brown. (Bake time - Note 5)

Rest & serve: Remove from the oven, rest for 5 minutes. Scoop into bowls and serve with vanilla ice cream or custard!