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Heat oven to 190C/170C fan/gas 5. Arrange the rhubarb pieces in a large roasting tin. Sprinkle over the sugar, orange zest and juice, grenadine and 150ml water. Cover with foil and bake for 25-30 mins until tender but not mushy. Scoop the rhubarb out of the tin into a sieve and reserve any juices, then leave to cool completely. Tip the rest of the liquid into a saucepan, along with the collected juices, and bring to the boil. Simmer for 15-20 mins or until reduced and syrupy. Leave to cool completely.
To make the crumble topping, rub the butter into the flour until the mixture resembles fine crumbs, then stir through the demerara sugar, almonds and oats. Tip onto a baking tray lined with parchment and bake for 15 mins, shaking the tray every so often, until golden brown, then cool.
Mix the Disaronno and orange juice together in a shallow dish. Dip each sponge into the liquid, leaving it for about 15 secs on each side, then arrange them in the base of a large trifle dish. Tip the mascarpone into a bowl and whisk until soft and smooth, then add the custard, a little at a time, and mix until combined. Arrange half the rhubarb pieces on top of the sponges, discarding the orange zest. Drizzle over 2 tbsp of the rhubarb syrup and top with half the custard mix. Follow with another layer of rhubarb, 1 tbsp of the syrup, and the remaining custard.
Whip the cream and 2 tbsp sugar to soft peaks and dollop over the custard. Drizzle over the remaining rhubarb syrup, along with a scattering of the crumble topping. Tip any extra crumble into a bowl and pass around the table.