Preheat the oven to 250 F.
Drop the egg whites in a clean dry bowl and using a hand mixer or stand mixer with a whisk attachment beat on a low speed until firm peaks form.
Increase the speed and start adding sugar, one tablespoon at a time making sure sugar is fully dissolved into the beaten egg whites before adding another tablespoon.
Beat sugar on a medium speed until stiff peaks form. (If you lift the whisk, the peaks will hold without collapsing).
Make sure not to overbeat the egg whites.
When all sugar has been incorporated and the meringue holds its shape and glossy, fold in cornstarch, lemon juice and vanilla.
Line a baking tray with a parchment paper and draw 8" circle in the middle.
Turn the parchment paper on the other side and spoon the meringue into the circle making a slight indentation in the center.
Bake Pavlova for 45-50 minutes until the outside looks dry to the touch.
Turn off the oven and without opening the door, let Pavlova completely cool before taking it out of the oven.
Meanwhile in a large bowl beat together heavy cream, Mascarpone cheese, vanilla and sugar.
Remove Pavlova from the oven and spoon Mascarpone cream into the indentation. Top with fresh peaches or nectarines and serve!