Make the pavlovas:

1. Combine the sugar and cornstarch in a small bowl.
2. Beat the whites and salt using an electric mixer until soft peaks form. Add water (whites will loosen) and beat until whites again hold soft peaks. Increase speed to medium-high and add the sugar mixture 1 tablespoon at a time, continuing 1 minute after all sugar is added. Add vinegar and beat at high speed until meringue is glossy and stiff peaks form, about 5 minutes.
3. Using a spoon or a spatula drop meringue on a parchment paper-lined baking sheet and use the back of a spoon to make a crater to put the curd and fruit in.
4. Bake at 300°F using convection oven or on the middle rack of a normal oven for 45 minutes, until pale golden and a thin crust forms (inside will still be soft). Turn oven off and prop the door open slightly with a wooden spoon. Let cool for one hour. Serve the same day with lemon curd (recipe below) and fresh berries. Meringue can also be frozen for up to two days if wrapped well in plastic wrap. Thaw before serving.

Make the lemon curd:

1. Heat lemon juice over medium heat until hot but not boiling.
2. Meanwhile, whisk eggs and yolk in a medium bowl. Gradually whisk in the sugar. While whisking, slowly pour hot lemon juice into the eggs. Return to saucepan and cook over medium heat. Stir constantly with a wooden spoon until mixture registers 170 degrees and coats the back of a spoon.
3. Remove pan from heat and stir in butter. Once melted, stir in cream, vanilla, and salt.
4. Strain mixture through a fine-mesh strainer and pour over pavlovas. The curd is able to be stored in an airtight container in the fridge for up to a week, just cover with plastic wrap to prevent it from drying out.