To make the pastry, measure the flour, butter and sugar into a food processor. Whiz until the mixture resembles fine breadcrumbs. Add the egg and whiz again until the dough comes together to form a ball. Turn out on to a lightly floured work surface and roll thinly. Carefully line the tin and press the pastry into the sides. Prick the base with a fork and place in the fridge to chill for 30 minutes.
Preheat the oven to 200°C/Fan 180°C/Gas 6 and place a baking tray in the oven to get very hot. Line the pastry case with non-stick baking paper, add baking beans, place on the hot baking tray and bake for 15 minutes. Remove the beans and paper and bake for another 5 minutes, or until the pastry is crisp and lightly golden. Leave to cool.
Reduce the oven temperature to 150°C/Fan 130°C/Gas 2. To make the filling, measure the cornflour and 200ml water into a pan and whisk to combine. Add the zest and juice of the lemon and lime and place over a medium heat. Continue to whisk until the mixture has boiled and thickened. Remove from the heat, add the sugar and egg yolks and whisk again. Pour into the pastry case and place in the fridge to chill.
Meanwhile, make the meringue topping. Place the egg whites in a large bowl and whisk using an electric whisk, until stiff. Add the sugar a little at a time, whisking on full speed, until you have a shiny, glossy meringue. Spoon into a piping bag fitted with a plain 1.cm nozzle and pipe even blobs over the surface of the chilled custard in a neat pattern. If you don’t have a piping bag, you can use two dessertspoons.
Bake in the oven for 35–40 minutes, until pale golden on top and firm to touch. Leave to cool in the tin for 15 minutes before removing from the tin. Serve warm with pouring cream.