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Heat the oven to 160C/140C fan/gas 4. For the shortbread, butter a 20cm shallow cake or tart tin. Whisk the butter and sugar together until pale and well-combined, then fold in both gingers, the pistachios and a pinch of salt. Sift in the flour and knead to a soft dough in the bowl. Tip into the tin and press the mixture into the base in an even layer.
Bake for 25-30 mins until pale golden and dry-looking. Cut the shortbread into slices or wedges while hot, then leave to cool in the tin – it will firm up as it cools. Sprinkle with the ½ tbsp caster sugar.
To make the mousse, whip the double cream, lemon zest, lemon curd and yogurt in a bowl with an electric whisk until soft peaks form. Spoon into small serving glasses or ramekins and chill.
Mash half the raspberries with the icing sugar using a fork, or purée in a small blender along with any defrosting juices to make a coulis. Cover and chill until ready to serve.
To serve, spoon the coulis over the mousses, garnish with the remaining raspberries and serve with the shortbread wedges alongside.