For the pavlova, preheat the oven to 100C/80C Fan/Gas ¼ and line a large baking tray with baking parchment.
Beat the egg whites with the caster sugar until stiff peaks form when the whisk is removed. Add the icing sugar and continue to beat for 4-6 minutes, or until the mixture is smooth and shiny.
Spoon the mixture onto the prepared baking tray creating a circle shape approximately 30cm/12in wide.
Bake for 2 hours. Once cooked, allow the pavlova to cool slightly before removing from the baking tray using a palette knife. Transfer to a cooling rack and set aside.
For the passion fruit and lemon posset, slowly bring the double cream and sugar to the boil in a large saucepan over a low heat, stirring well to dissolve the sugar in the cream. Boil for 3 minutes, then remove from the heat and set aside to cool.
Once the cream mixture has cooled, whisk in the lemon juice, zest and passion fruit juice and seeds. Pour the posset mixture into a glass bowl and chill in the fridge for at least 3 hours.
Whip the double cream until soft peaks form when the whisk is removed. Fold the posset into the cream (you can leave streaks of posset if desired).
To serve, spoon the cream and posset mixture on the top of the pavlova and decorate with the passion fruit seeds and lemon verbena.