In a bowl, combine the condensed milk and 1/3 cup of lemon juice.
Add lemon juice, stirring through completely - until when you draw a line through the mixture, the mixture does not creep in to fill in the hole made - but retains a firmish texture.
This may require adding more than 1/3 cup of lemon juice, depending on the strength of the juice you use. You can substitute fresh lemons for bottled pure lemon juice if preferred.
Pour into cooked pastry shell, and using a spatula to push the mixture right to the sides of the pastry and achieve a level appearance.
To top this, you can either spread some passionfruit pulp; sprinkle some crushed nuts; sprinkle hundreds and thousands; or decorate with whipped cream. The choice is yours.