Line and grease a 20cm (8-inch) springform cake tin with baking parchment.
Mix crushed digestive biscuits with melted butter until evenly coated.
Press the mixture firmly into the base of the tin and chill for 20 minutes.
Melt the white chocolate over gently simmering water and leave it to cool slightly.
Beat the cream cheese, icing sugar, vanilla, and soured cream until smooth.
Whip the double cream until soft peaks form.
Fold the melted white chocolate into the cream cheese mixture, then gently fold in the whipped cream. Add lemon zest or stem ginger if using.
Pour the filling onto the chilled biscuit base and smooth the top.
For the no-bake version: Refrigerate for at least 4 hours, preferably overnight.
For the baked version: Mix in the eggs, bake at 160°C (Fan 140°C/Gas 3) for 40–45 minutes, cool completely, then chill overnight.
Decorate with white chocolate curls, cocoa powder, raspberries, and strawberries before serving.