Preheat your oven to 375°F. Lightly butter a baking dish (9x13-inch or similar 8 ½ cups capacity).

Trim the 900 g fresh rhubarb (trimmed and cut into 2.5 cm (1 inch) pieces) and cut it into roughly 1-inch pieces. Place in the baking dish. Sprinkle over the 100 g caster sugar, 1 tbsp cornstarch, 1 tsp vanilla extract, and 1 tsp orange zest (if using). Toss everything together directly in the dish until the 900 g fresh rhubarb (trimmed and cut into 2.5 cm (1 inch) pieces) is evenly coated.

Spread the 900 g fresh rhubarb (trimmed and cut into 2.5 cm (1 inch) pieces) out into an even layer and set aside while you make the crumble topping. The 100 g caster sugar will begin to draw out the juices — this is exactly what you want.

In a large mixing bowl, combine the 150 g plain all-purpose flour, 100 g rolled oats, 100 g light brown sugar, ½ tsp ground cinnamon, and ¼ tsp fine salt. Stir together with a fork.

Add the cold cubed 120 g unsalted butter to the 150 g plain all-purpose flour mixture. Using your fingertips, rub the 120 g unsalted butter into the dry ingredients until the mixture resembles coarse, clumpy breadcrumbs with some pea-sized lumps remaining. Don't overwork it — those lumps create the best crunch. Alternatively, pulse a few times in a food processor.

Scatter the crumble topping evenly over the 900 g fresh rhubarb (trimmed and cut into 2.5 cm (1 inch) pieces). Do not press it down — a loose topping bakes up crispier. Sprinkle over the 1 tbsp demerara sugar if using, for extra crunch.

Bake in the preheated oven for 35–40 minutes, until the topping is deep golden brown and the 900 g fresh rhubarb (trimmed and cut into 2.5 cm (1 inch) pieces) juices are bubbling up around the edges. If the top colours too quickly, loosely tent with aluminum foil for the last 10 minutes.

Remove from the oven and allow to rest for at least 10 minutes before serving — the filling will thicken as it cools slightly. Serve warm with 1 tsp vanilla extract ice cream, clotted cream, or warm custard.