Preheat the oven to 350°F (175°C). Line a baking sheet with parchment paper and, if desired, trace an 8–9 inch circle on the underside of the parchment as a guide.
Fill a medium pot with about 1 inch of water and bring to a very gentle simmer over low heat.
In a heatproof bowl, combine the room-temperature egg whites and granulated sugar. Place the bowl over the simmering water (ensure the bowl bottom does not touch the water) and whisk constantly until the sugar dissolves and the mixture is warm to the touch.
Remove the bowl from the heat and transfer to a stand mixer fitted with the whisk (or use a hand mixer). Beat until medium-stiff peaks form.
Gently fold in the cornstarch, then add the lemon juice and vanilla extract and whisk just until stiff peaks form and the meringue is glossy.
Spoon the meringue onto the prepared parchment inside the traced circle. Spread it into an even round and use the back of a spoon to create a shallow well in the center and shape the edges as desired.
Place the baking sheet in the oven and immediately reduce the heat to 300°F (150°C). Bake for 1 hour, then turn off the oven and leave the pavlova inside to cool completely as the oven cools (several hours or overnight is fine).
Just before serving, whip the chilled heavy cream with the powdered sugar until soft peaks form.
Top the cooled pavlova with the whipped cream and arrange berries and other fruit on top. Serve immediately.