Make the pastry: Put all the pastry ingredients, along with 1 tablespoon cold water into a food processor and pulse until everything is combined. Tip onto a floured work surface and roll out to 4-5mm thick. Line your pie dish and trim the sides. Prick the base and pop into the freezer to chill for an hour.
Fill with baking beans and bake for 10 minutes, then remove the beans and cook for a further 5 minutes. Remove from the oven and leave to cool.
Make the white layer: Mix together the mascarpone, sugar and vanilla and spread into the cooled pastry case. Smooth the top.
Make the meringue: Whisk the egg white to stiff peaks with an electric whisk. Slowly add the sugar, a teaspoon at a time. When all the sugar has been added, whisk through the yellow food colouring and lemon flavouring.
To serve:Dollop the meringue onto the pie case, making nice shapes with the back of a spoon. Blowtorch the top to highlight the peaks, then slice and serve.