Heat the oven to 190°C conventional, 170°C fan, or gas mark 5. Wash and trim the rhubarb thoroughly, removing every leaf, then cut the stalks into evenly sized 2–3 cm pieces.

Place the prepared rhubarb in a shallow baking dish and toss it with the sugar and 1 tablespoon of flour. Add the orange zest and orange juice or water if the stalks seem particularly dry.

To make the topping, rub the cold cubed butter into the flour and salt with your fingertips until the mixture resembles coarse breadcrumbs. Stir in the sugar and rolled oats, if using.

Scatter the crumble mixture evenly over the rhubarb. Keep the topping loose rather than pressing it down, since the spaces help the buttered crumbs bake into a crisp, textured crust.

If time allows, chill the assembled crumble for 10–15 minutes before baking. This keeps the butter cold and encourages a more textured topping.

Bake for 35–40 minutes, checking toward the end of the cooking time. The crumble should be golden brown, and the rhubarb should be tender with juices bubbling around the edges.

Remove the dish from the oven and let it rest for 10 minutes. This short pause allows the bubbling fruit juices to settle slightly while the topping remains pleasantly crisp.

Spoon the warm crumble into bowls and serve with custard, vanilla ice cream, or pouring cream. Divide the dish into six servings for a generous dessert.