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Line a 900g loaf tin with baking parchment. Put the chopped rhubarb in a medium-sized saucepan with the caster sugar, ginger cordial and 2 tbsp water. Heat gently until the sugar has dissolved, then simmer for 10-15 mins or until the rhubarb is soft but still holds its shape. Pour into a heatproof jug and leave to cool.
In a large bowl, whisk the egg whites with an electric hand whisk until stiff, then set aside. In another bowl, whisk the yolks with the icing sugar until they’re pale and starting to thicken. Finally, in a third bowl, whip the double cream until thick. Fold the yolk mixture into the whipped cream until well combined, then fold in the egg whites and half of the broken meringues.
Gently spoon a third of the cream mixture into the prepared tin and freeze for 20-30 mins or until just set (keeping the rest of the cream mixture in the fridge). Once it has set, take the tin out of the freezer and pour in half of the cooked rhubarb. Reserve a little of the cooked rhubarb for decorating later, if you like. Top with another third of the cream mixture and return to the freezer for another 30 mins. Once this layer has set, add the remaining rhubarb, followed by a final layer of cream. Cover with cling film and put it back in the freezer for 2 hrs to firm up.
To serve, turn it out onto a serving plate and peel away the parchment. Decorate with any reserved rhubarb, the remaining meringue pieces and the crystallised ginger. Slice and serve immediately.