To make your pastry cases visit Sweet Pastry
Heat the oven to 160C.
Mix 2 level tbsp cornflour, 100g golden caster sugar and finely grated zest of 2 large lemons in a medium saucepan. Strain and stir in 125ml lemon juice gradually.
Make the juice of 1 small orange 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 85g butter until melted. Beat 3 egg yolks (save white for meringue) and 1 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 3 egg whites in a large bowl. Whisk to soft peaks, then add the golden caster sugar and cornflour a spoonful at a time, whisking between each addition without over beating, whisk until smooth and thick.
Fill the tart case with the filling, you may need to warm it up again. Pipe on the meringue or whatever way you want.
Bake for 20 mins.