Put ¾ of the passion fruit pulp and seeds (reserve the rest for decorating) in a food processor or blender and whiz to a purée. Push through a sieve into a bowl, to remove the seeds. Add the condensed milk and stir until smooth.
Pour the cream into another bowl and use a hand-held electric whisk to whip until stiff peaks form. Fold through the passion fruit mixture, a third at a time, until combined.
Spoon into small serving glasses and chill for at least 1 hour before serving. Spoon the reserved passion fruit over the mousses and serve with the lemon thins.