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

Trim the 31 ¾ 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 3 ½ g caster sugar, 1 tbsp cornstarch, 1 tsp vanilla extract, and 1 tsp orange zest from about half an orange (if using). Toss everything together directly in the dish until the 31 ¾ g fresh rhubarb trimmed and cut into 2.5 cm (1 inch) pieces is evenly coated.

Spread the 31 ¾ 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 3 ½ g caster sugar will begin to draw out the juices — this is exactly what you want.

In a large mixing bowl, combine the 5 ¼ g plain all-purpose flour, 3 ½ g rolled oats old-fashioned oats, 3 ½ g light brown sugar, ½ tsp ground cinnamon, and ¼ tsp fine salt. Stir together with a fork.

Add the cold cubed 4 ¼ g unsalted butter cold to the 5 ¼ g plain all-purpose flour mixture. Using your fingertips, rub the 4 ¼ g unsalted butter cold 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 31 ¾ 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 for sprinkling on top if using, for extra crunch.

Bake in the preheated oven for 35–40 minutes, until the topping is deep golden brown and the 31 ¾ 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.