Preheat the oven to 100°C and line a large baking tray with baking parchment.
Beat egg whites with caster sugar until stiff peaks form, then beat in icing sugar for 4-6 minutes until shiny.
Spoon mixture onto the tray to create a 30cm circle.
Bake for 2 hours, cool slightly, then transfer to a cooling rack.
Bring 600ml double cream and sugar to a boil slowly, boiling for 3 minutes before cooling.
Whisk in lemon juice, zest, and passion fruit into the cooled cream, then chill for 3 hours.
Whip remaining 600ml double cream to soft peaks and fold in the chilled posset.
Spoon the mixture over the pavlova and decorate with remaining passion fruit and lemon verbena.