- Preheat the oven to 100 °C (200 °F). Line a baking sheet with parchment paper. With a pencil, draw a 25-cm (10-inch) in diameter circle on the paper.
- In a bowl, beat the egg whites and cream of tartar with an electric mixer until soft peaks form. Add the sugar and beat until stiff peaks form. Spoon the meringue onto the parchment paper to form a 25-cm (10-inch) in diameter disk.
- Smooth down the centre of the meringue to give it the shape of a pie plate. The bottom must be at least 2.5-cm (1-inch) thick. Bake for 4 hours and 30 minutes (the meringue will lightly brown). Turn the oven off and leave the meringues to dry overnight. If it is still tacky the next day, dry 1 more hour in the oven.
- In the upper part of double boiler, off the heat, whisk together the egg yolks and sugar until frothy. Add the lemon juice, butter and water. Place over simmering water and stir constantly until it slightly thickens. Dissolve the cornstarch in the cold water. Stir into the lemon mixture. Cook until it thickens. Cover the surface with plastic wrap and refrigerate. To serve, spread the curd in the centre of the meringue. Garnish with seasonal berries.
The lemon curd is delicious on vanilla ice cream, berries or as a filling for lemon pies.