
Preheat the oven to 200°C (180°C fan).
In a large bowl, gently toss together the rhubarb, orange zest and juice, caster sugar, cornflour, vanilla and chopped stem ginger. Transfer to a baking dish about 25cm wide and spread evenly.
To make the crumble topping, place the flour and cold butter in a bowl and rub together with your fingertips until the mixture resembles coarse breadcrumbs.
Stir in the demerara sugar, oats, hazelnuts, ground ginger and mixed spice until evenly combined.
Scatter the crumble evenly over the rhubarb without pressing down.
Bake for 35–40 minutes, until the topping is deep golden and the fruit is bubbling around the edges.
Leave to rest for 10–15 minutes, then serve warm with ice cream, cream or custard.