For the Lemon Aspen Semi Freddo, place quandongs, 150g of the sugar, 250ml water and lemon aspen juice in a small saucepan over a medium heat. Remove from heat and use a stick blender to blitz until smooth. Return to medium heat and cook until reduced until syrupy. Remove from the heat and pass through a fine sieve, discard solids and set aside 2 tablespoons of the syrup in a bowl for garnish. Set remaining syrup aside to cool.
Place the egg yolks and remaining sugar in a medium saucepan and whisk together until thickened and pale. Add milk and place over a low heat, stirring constantly, until mixture thickens and coats the back of a spoon. Remove from heat aside to cool.
Meanwhile, whip the cream to soft peaks. Gently fold the cream through the cooled egg mixture until fully combined. Sit 4 x 4cmx9cm rectangular silicone moulds on a baking tray. Pipe the cream mixture into the moulds to ¾ full. Place into the freezer until just set, about 1 hour. Remove from the freezer and pour the cooled lemon verbena and quandong syrup into the moulds of cream mixture to fill to the top. Return to the freezer until firm.
Meanwhile, for the Lemon Aspen Crumb, place butter and sugar in the bowl of stand mixer fitted with a paddle attachment. Beat on medium speed until light and fluffy. Add egg yolk and beat until combined, then add flour and lemon aspen juice and mix well until mixture comes together.
Lightly flour a clean work surface. Transfer the mixture to the floured surface and knead gently, adding more flour if required to form a soft, smooth dough. Line a baking tray with baking paper. Spread dough out onto the lined baking tray and press down with fingertips to form an even layer. Bake in the oven until golden, about 12-15 minutes. Remove from oven and use a fork to break into crumbs then set aside to cool.
For the Fresh Cheese Mousse, place fresh cheese and sugar in a small saucepan and heat over low heat until sugar has dissolved. Remove from the heat and set aside to cool. Meanwhile, whisk cream until slightly thickened. Add cooled cheese mixture to the thickened cream and continue whipping to soft peaks.
To serve, brush a line of the reserved lemon verbena and quandong syrup across each plate. Unmould Lemon Aspen Semi Freddo and place, syrup side down, across the line of syrup. Pipe mounds of the Fresh Cheese Mousse beside the line of syrup and on top of the Lemon Aspen Semi Freddo. Sprinkle the mounds of mousse with the Lemon Aspen Crumb. Finish the Semi Freddo with a little more of the reserved quandong syrup and some sprigs of Geraldton wax flowers.