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For the pastry, put the plain flour, butter, icing sugar, egg yolk (save the white for the meringue) and 1 tbsp cold water into a food processor. Pulse until the mix starts to bind – make sure the mix is not overworked.
Tip the pastry onto a lightly floured surface, gather together until smooth, then roll out and line a 23 x 2.5cm loose-bottom fluted flan tin. Trim and neaten the edges. Press pastry into the flutes. The pastry is quite rich, so don’t worry if it cracks, just press it back together. Prick the base with a fork, line with foil, shiny side down, and chill for 30 mins-1 hr (or overnight).
Put a baking sheet in the oven and heat the oven to 200C/180C fan/gas 6. Bake the pastry case ‘blind’ (filled with dry beans) for 15 mins, then remove the foil and bake a further 5-8 mins until the pastry is pale golden and cooked. Set aside. Can be done a day ahead. Lower the oven to 180C/160C fan/gas 4.
While the pastry bakes, prepare the filling. Mix the cornflour, golden caster sugar and lemon zest in a medium saucepan. Strain and stir in the lemon juice gradually. Make the orange juice up to 200ml with water and strain into the pan. Cook over a medium heat, stirring constantly, until thickened and smooth.
Once the mixture bubbles, remove from the heat and beat in the butter until melted. Beat the egg yolks (save white for meringue) and the whole egg together, stir into the pan and return to a medium heat. Keep stirring vigorously for a few minutes, until the mixture thickens and plops from the spoon. (It will bubble, but doesn’t curdle.) Take off the heat and set aside while you make the meringue.
Put the egg whites in a large bowl. Whisk to soft peaks, then add 100g of the golden caster sugar a spoonful at a time, whisking between each addition without overbeating. Whisk in the cornflour, then add the remaining 100g of sugar as before until smooth and thick.
Quickly reheat the filling and pour it into the pastry case. Immediately put spoonfuls of meringue around the edge of the filling (if you start in the middle, the meringue may sink), then spread so it just touches the pastry (this will anchor it and help stop it sliding). Pile the rest into the centre, spreading so it touches the surface of the hot filling (and starts to cook), then give it all a swirl.
Return to the oven for 18-20 mins until the meringue is crisp and slightly coloured. Let the pie sit in the tin for 30 mins, then remove and leave for at least another 30 mins-1 hr before slicing. Eat the same day.