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Make the semi-freddo. Push 140g/5oz of the raspberries through a sieve, leaving behind the seeds. Stir in 25g/1oz of the icing sugar. Whip the cream until like thick custard. Whisk in the mascarpone until the mix thickens, then stir in the remaining icing sugar.
Drop spoonfuls of the cream mix into a deep, oblong plastic container, and fill in with a drizzle of sieved raspberries. Do not stir. Add another layer of cream and raspberries. Gently smooth the surface and press down lightly, then freeze for 2-21⁄2 hours until half frozen.
Heat oven to 140C/fan 120C/gas 1. Line a large baking sheet with non-stick baking parchment. Draw a 20-22cm circle on the paper. In a large bowl, whisk the egg whites until stiff peaks form. Slowly whisk in the caster sugar until thick and glossy. Beat in the cornflour. Pile the meringue in soft folds onto the circle on the baking parchment. Scatter over the pistachios. Bake for 11⁄4-11⁄2 hrs until crisp on the outside and dry underneath. Cool then gently peel off the paper.
Remove the semi-freddo from the freezer and pile in big spoonfuls on the pavlova. Finish off with a scattering of the remaining raspberries.