14 oz (400 g) shortcrust pastry
3 tbsp cornstarch
4 large lemons
2 tbsp butter, diced
1 1/4 cups sugar
1/2 tsp cream of tartar
4 large eggs, separated
1/2 tsp pure vanilla extract
Preheat the oven to 400°F (200°C). Fit the pastry into a 9in (23cm) tart pan with a removable bottom, fitting it tightly into the corners, and trim the excess dough.
Prick the dough with a fork. Line with wax paper and fill with baking beans. Chill for 30 minutes.
Place on a baking sheet and bake for 10 minutes, until the dough looks set.
Remove the paper and the beans and bake until the dough is crisp and golden brown, about 10 minutes more. Transfer to a wire rack.
Reduce the oven temperature to 350°F (180°C).
Grate the zest from 2 of the lemons. Juice the lemons: you should have 3/4 cup of juice.
Whisk 3/4 cup of the sugar, the egg yolks, and cornstarch together in a bowl until the mixture is pale yellow.
Gradually whisk in 1 cup water and the lemon juice. Add the butter. Pour into a saucepan.
Bring to a simmer over medium heat, stirring constantly, until simmering and thickened. Reduce the heat to low and let bubble for 30 seconds. Strain through a wire sieve. Stir in the zest. Spread evenly in the tart shell.
Beat the egg whites in a large bowl with an electric mixer until foamy. Add the cream of tartar and beat until soft peaks form. Beat in the remaining 1/2 cup sugar, a tablespoon at a time, until the meringue is thick and glossy.
Spread the meringue over the hot filling, being sure that it touches the pie crust on all sides.
Place the tart on a baking sheet. Bake for 12–15 minutes until the meringue is tinged light brown. Transfer to a wire rack and let cool completely. To slice, dip a thin, sharp knife into a tall glass of hot water before each slice.