Pavlova with Lemon Creme, Berries and Grapes

For the meringue...
1 cup superfine granulated sugar
1 tablespoon cornstarch
3 large eggs at room temperature 30 minutes
3 tablespoons cold water
1 teaspoon distilled white vinegar
For the filling...
2/3 cup granulated sugar
1 tablespoon cornstarch
1/8 teaspoon salt
1/3 cup fresh lemon juice
1/2 stick unsalted butter
3 large egg yolks, lightly beaten
2 teaspoons grated lemon zest
1 cup heavy cream
2 cups mixed berries
2 cups grapes
Preheat oven to 300ºF and position a rack in the center.
To prepare the lemon cream, stir sugar, cornstarch, and salt in a saucepan. Add the lemon juice and butter, bringing the mixture to a simmer over medium high heat. Continue to whisk at a simmer, about 1 minute. Whisk about 1/4 of the mixture into the beaten egg yolks, then transfer the egg yolk mixture back into the saucepan. Over low heat, continue to cook, but make sure not to boil, whisking constantly until the lemon curd is thick, about 2 minutes. Scrape into a shallow bowl, stir in the lemon zest, and place a piece of parchment over the surface. Refrigerate for about 1-1/2 hours.
To prepare the meringue, line a baking sheet with parchment and trace a circle about 7" in diameter in the center. Turn the parchment over.
Whisk superfine sugar and cornstarch together in a small bowl. In the bowl of an electric mixer, beat whites with a pinch of salt at medium speed until soft peaks form. Add the water and beat until whites hold soft peaks once again.
On medium-high, beat in sugar mixture 1 Tbsp at a time. After all sugar has been added, beat 1 minute longer. Add vinegar, then beat at high speed until glossy and stiff peaks form, about 5 minutes (longer if using hand-held mixer). The meringue will be extremely thick.
Spread meringue carefully to cover the circle on the parchment, creating a cavity in the center (for the filling). Bake until meringue is pale golden and has a crust, about 45 minutes. Avoid opening the oven door! Turn oven off and prop door open slightly with a wooden spoon. Cool meringue in oven 1 hour. The exterior will be dry and possibly cracked, the inside more like the consistency of marshmallow.
To assemble the pavlova, beat the heavy cream just as it holds stiff peaks, then 1/4 cup at a time, whisk cream into the lemon curd. Check consistency each time before adding more cream. It should be able to mound. Spoon lemon cream into cooled meringue and mound fruit in the center. Serve with extra whipped cream if desired.