In a medium bowl, combine the milk, lemon verbena, lime leaves, lemon zest, orange zest and lime zest. Cover and steep at room temperature for 1 hour. Strain half of the milk through a fine-mesh sieve into a small saucepan and bring it to a simmer over low heat. In a small bowl, whisk together the egg yolks and sugar. Strain in the remaining half of the milk through a fine-mesh sieve and stir together. Slowly pour the simmering milk into the egg mixture while whisking. Return the mixture to the saucepan and gently whisk while heating to 180°F (82°C). Remove from the heat and add the white chocolate, orange liqueur and salt. Whisk until the chocolate is melted. Strain through a fine-mesh sieve into a medium bowl and cool to room temperature. In a separate medium bowl, whip the cream to soft peaks, then gently fold the whipped cream into the citrus mixture using a whisk. Scrape the mixture into a piping bag (no tip required) and pipe into eight 5-ounce semi-sphere silicone moulds (use about ½ cup per mould). Allow the semifreddo to partially set in the freezer for about 15 minutes. Pop the frozen citrus curd from the moulds and gently push the curd, curved side down, into the centre of the semifreddo. The bottom of the curd should be covered by the semifreddo mixture. Freeze until set, about 8 hours.