Bake a warm Apple and rhubarb crumble with a crunchy oat topping and serve it with vanilla ice cream.
Preheat the oven to 200°C/375°F (180°C fan). While this is running in the background, arrange your work area so the baking dish and mixing bowls are ready — this brief pause keeps the flow of the recipe tidy and lets you focus on the assembly that follows.
Place the rhubarb stems, the peeled and 1.5cm-diced Granny Smith apples, the white sugar and the orange zest in a large mixing bowl. Toss gently until the sugar and zest coat every cube and stalk, the fruit looking glossy and taut with juice. Spread this mixture evenly into a rectangular 30 x 20 cm (12 x 8 in) baking dish so the fruit layer is level and ready to receive the topping.
Combine the rolled oats, plain flour, tightly packed brown sugar, baking powder, cinnamon and a pinch of salt in a medium mixing bowl. Use a wooden spoon and a cutting-and-smearing motion to work the dry ingredients together until the texture is uniformly sandy and flecked with oat fragments.
Pour the melted unsalted butter into the dry mix and stir until evenly moistened — the mixture should hold together when pinched but still be loose and sandy rather than paste-like. With handfuls, scrunch and enclose your fist, pressing the mixture into lumps, then break those lumps apart and scatter the irregular clumps and oat flakes across the top of the fruit in the rectangular dish so the filling peeks through in places.

Place the prepared, topped baking dish into the oven and bake for about 35 minutes, or until the rhubarb is soft and the crumble is a deep golden brown with toasted oats visible on the surface. The fruit below should be glossy and syrupy, the topping crisp and lightly clumped.
Remove the dish and let it rest for about 5 minutes so juices settle. Scoop warm portions into bowls and serve immediately with scoops of vanilla ice cream or warm pouring custard so the cold cream melts slightly into the hot fruit, creating ribbons of cream across the crumble.
