2 cups mixed fresh berries (strawberries sliced, if using)
3 tablespoons sugar
1 teaspoon finely grated lemon zest
1 cup heavy cream
Set a rack in the center of the oven and preheat to 300°F. Trace a 6-inch circle on a sheet of parchment paper and invert the paper onto a baking sheet so the drawn side is face down (you should still be able to see the circle).
Whisk the sugar and cornstarch in a small bowl. Combine the egg whites and a pinch of salt in the bowl of a stand mixer fitted with the whisk attachment (or use a bowl and an electric hand mixer). Beat on medium speed until the whites hold soft peaks, 1–1 ½ minutes. Raise the speed to medium-high and add the sugar-cornstarch mixture 1 tablespoon at a time, then beat 1 additional minute, Add the vinegar, raise the speed to high, and beat until the mixture is shiny and holds stiff peaks, 4–5 minutes.
Carefully pour the meringue onto the circle on the sheet pan, using a silicone spatula to gently spread it inside the borders. Bake until the meringue is lightly golden and develops a soft shell, 40–45 minutes. Do not remove from the oven!
Turn off the oven and wedge a wooden spoon or other narrow stick in the door to keep the oven open; let sit in the oven until the meringue hardens, 1 hour.
Toss the berries with 1 tablespoon of sugar and the lemon zest in a medium bowl. Let them sit (this is called macerating) while you whip the cream.
Using the stand mixer (or hand mixer again), beat the cream and remaining 2 tablespoons sugar until the cream holds soft peaks, 2–3 minutes.
Remove the pavlova from the parchment and place on a serving plate or platter. Dollop the cream onto the pavlova, leaving a 2-inch border around the edges. Top with the berries and slice into wedges.