Beat the egg whites with a mixer until they reach stiff peaks.
Gradually add the granulated sugar, lemon juice and cornstarch, mixing until the meringue is glossy and thick.
Transfer the meringue mixture to a piping bag and line a baking sheet with parchment paper.
Pipe small circles of meringue onto the baking sheet, creating low sides to form nests. Bake at 210°F (100°C) for about 2 hours, until dry and crisp on the outside.
Prepare the cream: beat the cold heavy cream with powdered sugar until it thickens, then add the mascarpone and beat again just until smooth and fluffy.
Once the meringue bases are baked, let them cool completely. Spoon or pipe the mascarpone cream into a piping bag, use a metal ring or template if desired, and fill the cooled meringue nests with the cream.
Decorate the pavlovas with fresh berries and, if you like, a few small mint leaves, then dust lightly with powdered sugar before serving.