Preheat the oven to gas 4, 180°C, fan 160°C. Cut a thick strip from a short end of the pastry rectangle and patch it onto one of the long sides to make a square, pressing down to seal the join. Roll the pastry sheet on a lightly floured surface to make it a little larger. Press into a 23cm (9in) diameter, 8cm (3in) deep cake or flan tin, making sure the pastry reaches right up the sides.
Chill for at least 30 minutes then trim the sides evenly. Cover the pastry with a sheet of nonstick baking paper and line with baking beans. Bake blind for 10 minutes, then remove the beans and paper and cook for 8 minutes more, until pale golden.
Meanwhile, pour the tangerine juice into a measuring jug with the lemon juice. It should reach the 600ml (1pt) mark. If not, make up the extra with water. Mix the cornflour and sugar in a large saucepan and gradually add about 100ml (3½fl oz) citrus juice, while stirring, to make a smooth paste. Now stir in the remaining citrus juice and add the tangerine zest. Put the saucepan over a low heat and slowly bring the mixture to the boil, stirring to prevent lumps, until it thickens. Keep cooking the mixture for a couple of minutes after it has boiled, stirring constantly. Remove from the heat and stir in the butter until smooth. Now stir in the egg yolks. Transfer the cooked pastry case from its tin to a baking sheet and fill with the tangerine mixture.
To make the meringue, whisk the egg whites in a clean bowl to stiff peaks. Gradually whisk in the sugar. Spread the mixture (which should be thick and glossy) over the tangerine filling, completely covering the surface with no gaps between pastry and topping. Use the back of a spoon to create peaks and swirls.
Bake for 15 minutes, or until the meringue peaks are golden. Serve with cream or ice cream.
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