In a small saucepan, whisk together sugar, cornstarch, and salt. Add the cherries and raspberries, and bring to a boil over medium-high heat. Boil for one minute, then stir in lemon juice and elderflower liqueur.

Transfer the cooked fruit into a food processor and blend until smooth. Pass this mixture through a sieve, discarding solids. Set aside about 310 ml of the purée for later.

In a large heatproof bowl, combine whole eggs, egg yolks, sugar, and salt. Set over simmering water and whisk until warm and sugar dissolves, reaching about 71°C.

Using an electric mixer, beat the egg mixture on medium-high speed until thickened and cooled, about 3 to 5 minutes.

Reduce the mixer speed to low and slowly beat in one cup of the berry purée.

In a separate bowl, whip heavy cream to stiff peaks, then gently fold it into the berry-egg mixture until combined.

Divide the berry mixture evenly into eight freezer-safe glasses. Freeze until firm, at least 8 hours or up to 3 days if covered with plastic wrap. Refrigerate the remaining berry purée in an airtight container.

Before serving, let the semifreddo stand at room temperature for 10 minutes.

Whip the remaining cream to soft peaks, then spoon the reserved berry purée over the semifreddo. Top with fresh cherries and raspberries.