Beneath the puffy dome of soft meringue is a double hit of citrus – first, there’s the lemon and then you get the sweet whammy of freshly squeezed orange juice, too. Chilling tempers flavour, so serve the pie warm to enjoy it at its best.
Make the pastry. Tip the flour, salt and butter into a mixing bowl. Using your fingertips, rub the butter into the flour until the mixture resembles fine breadcrumbs. Add the icing sugar and egg, then mix to combine. Bring together to form a rough ball of dough.
Make the pastry case. Lightly flour the work surface and roll the pastry to a disc large enough to line the base and sides of the tin and about 3mm thick. (This is easier between 2 sheets of baking paper.) Lift the pastry into the tin, being careful not to stretch it as you press it into the base and sides. Trim the excess and chill it for 30 minutes, until firm. Heat the oven to 200°C/180°C fan/400°F/Gas 6.
Line the chilled pastry case with baking paper and baking beans or rice. Bake on the middle shelf for 15 minutes, until the top edge starts to turn crisp and golden. Remove the baking paper and beans or rice. Return the case to the oven and bake it for a further 5–10 minutes to dry out and cook the base. Leave to cool.
Make the filling. While the pastry case is cooling, mix the lemon zest and juice with the cornflour in a small bowl until smooth. Pour 150ml of water and the orange juice into a saucepan and bring it to the boil. Add the lemon mixture and simmer, stirring, until thick enough to coat the back of the spoon, then remove the pan from the heat.
In a large bowl, mix the caster sugar and egg yolks together, then pour in the hot lemon mixture, stirring continuously. Pour the mixture back into the pan and stir over a medium heat to a thick custard. Remove from the heat and leave to cool slightly, then pour the custard into the baked pastry case. Chill until set.
Make the meringue. Heat the oven to 150°C/130°C fan/300°F/Gas 2. Whisk the egg whites and cream of tartar in a stand mixer fitted with the whisk until soft peaks form. A spoonful at a time, add the caster sugar, whisking continuously until the meringue is stiff and glossy. Spoon the meringue in a large dome on top of the set lemon filling, creating swirls with the back of a spoon. Bake for 15 minutes, until the meringue is pale golden and crisp. Serve warm.