Preheat the oven to 200°C/Gas Mark 6. In a food processor, blitz the ginger biscuits to crumbs. Add the apple sauce and pulse until the mixture comes together. Tip into a 36x12cm (15x5in) loose-bottomed tart tin and press into the corners and up the sides. Bake for 10 minutes. Remove and reduce the temperature to 180°C/Gas Mark 4.
Wash the lemon, then chop into chunks, discarding any seeds. Wipe the food processor clean, then add the lemon. Process until it has completely broken down, then add the lemon juice, 125g (4½oz) sugar, the cornflour and egg yolks. Process until you have a thick, pale yellow mixture.
Set a small heatproof bowl over a pan of simmering water. Pour the lemon mixture into the bowl and stir continuously for 5 minutes, until heated through. Pour into the baked biscuit case, then transfer to a baking tray.
Whisk the egg whites until soft peaks form. Gradually add the remaining sugar, whisking constantly, until you have a thick, glossy meringue. Spoon into a piping bag fitted with a star-shaped nozzle and pipe 12 rows on top of the tart.
Bake for 25 minutes until the meringue is golden. Remove from the oven and let cool for 3 hours, then slice and serve.