For the raspberry sorbet, add the sugar and water to a saucepan. Heat the mixture gently until the sugar dissolves then bring to a boil and immediately take off the heat then set aside to cool.
Meanwhile, add the raspberries to a food processor along with the lemon juice and process to a smooth purée. Add 250ml of the syrup to the fruit purée then taste and add more of the syrup if the mixture is too tart. Chill the mixture in the fridge then pour the mixture into the bowl of an ice cream machine and churn according to the manufacturer's instructions.
Place sifted flour and butter with salt unto a food processor and blitz until combined. Add enough egg until the mixture forms a dough. Remove from food processor and combine. Wrap in cling film and rest in the fridge for minimum 30 minutes.
Roll out the pastry to about 5mm thick and use to line a 28cm baking mould. Chill for 30 minutes.
Preheat oven to 180°C. Line mould with greaseproof paper and fill with baking beans. Bake for 15 minutes. Remove paper and beans and bake for a further 5 minutes or until the base is slightly golden. Remove from the oven and brush lightly with egg yolk to seal and allow to cool. Remove the pastry case from the mould and set aside.
For lemon filling place the cream, lemon zest and vanilla pod in a pan and bring to the boil. Remove from the heat and infuse for 30 minutes. Add the lemon juice to the cream. Whisk together the egg yolks and sugar in a bowl. Strain the cream mixture over, whisking constantly.
Turn the oven to 150°C. Pour the custard filling into the pastry case and place in the oven. Bake for 35-40 minutes. Remove when centre is slightly wobbly. Put the tart into the fridge to set.
To serve, slice a wedge of the lemon tart and sprinkle the top with caster sugar. Blow torch the top just like a crème brûlée. Place the slice on a plate. Using an ice cream scoop just taken out of hot water scoop the raspberry sorbet out of the container. The warm ice cream scoop will give the sorbet a nice gloss as well as make it easier to carve through the sorbet.