Preheat the oven to 300 degrees F. With a marker, mark a 7-inch circle on a piece of parchment paper. Flip the parchment over and place on a baking sheet.
In a small bowl, whisk together the sugar and cornstarch.
In the bowl of a stand mixer, combine the egg whites with a pinch of salt and beat with the whisk attachment until soft peaks form. Add 3 tablespoons water and keep beating until you get soft peaks again.
Increase the mixer speed to high. Add the sugar mixture 1 tablespoon at a time and continue to beat for 1 minute. Add the vinegar and beat until the meringue is glossy and holds stiff peaks, about 5 minutes.
Spread the meringue inside the marked circle on the parchment, making the edges slightly higher than the inside of the circle (it should look like a "crater"). Bake until the meringue is pale golden and has a crust, about 45 minutes; the inside should still be marshmallow-like. Turn off the oven, prop the door open and let the meringue dry for 1 hour.
To serve, place the Pavlova on a serving dish. Spread the inside of the crater with a thin layer of lemon curd. Top with berries and serve.