Use a pencil to mark the outline of a 10” disc on a piece of parchment paper. Turn the paper over and lay it in a half sheet pan, then set aside.
Add the egg whites with a pinch of salt to a dry, clean bowl of your stand mixer. Using the whip attachment, begin whipping the egg whites to create a French meringue.
Once they have become very frothy and starting to turn white, add the sugar 1/4 cup at a time until it is all incorporated and has formed very stiff peaks. Next add the vinegar, vanilla and sifted cornstarch and continue beating until it is incorporated.
Pour this out onto your parchment paper and use an offset spatula to spread out the meringue. Bake this in a preheated 250 degrees F oven for 1 hour, and then turn off the oven and allow the meringue to cool completely before removing from the oven.
Once cooled, whip the cream with the powdered sugar and vanilla to stiff peaks. Feel free to add more sugar if you prefer. Top the entire Pavlova with the whipped cream.
In the meantime, rinse your berries and set aside. Add the apple jelly to a microwave safe bowl and heat it until melted.
Toss this into the berries and add them on top of the whipped cream. The apple jelly gives that beautiful shine to the berries and is perfect to use here. It seals and glazes the fruit beautifully. Serve immediately.
Note: You can make the meringue disc ahead of time and then decorate it right before service. If you put the disc together with the whipped cream too far in advance and refrigerate, the meringue will get soggy. So make sure to prepare the disc the day before and allow it to cool completely, then cover and store at room temperature until you are ready to put the cake together.