Preheat your oven to 250°F (120°C). Line a large baking sheet with parchment paper. In a clean, grease-free bowl, beat the egg whites with an electric mixer until stiff peaks form.
Slowly add the sugar, a tablespoon at a time, while continuing to beat on high speed. Beat until the meringue is glossy and smooth, and the sugar is completely dissolved.
Gently fold in the vanilla extract, white vinegar, and cornstarch. Be careful not to overmix.
Spoon or pipe the meringue onto the prepared baking sheet in a large circle, about 9-10 inches in diameter. You can create a slight well in the center to hold the filling later.
Bake for 1 hour and 15 minutes, or until the meringue is crisp on the outside and dry on the inside. Turn off the oven and let the meringue cool completely inside the oven with the door slightly ajar.
In a medium saucepan, whisk together the egg yolks, sugar, and cornstarch until smooth. Gradually whisk in the milk and heavy cream. Cook over medium heat, stirring constantly, until the custard thickens.
Remove from heat and stir in the vanilla extract and salt. Pour the custard into a bowl, cover with plastic wrap, and refrigerate until completely chilled.
In a medium saucepan, combine the mixed berries, sugar, and lemon juice. Cook over medium heat, stirring occasionally, until the berries break down and the sauce thickens slightly. Let the sauce cool completely.
In a large bowl, whip the heavy cream with the powdered sugar until stiff peaks form.
Gently transfer the meringue to a serving platter. Spoon the chilled custard into the center of the meringue. Top with the berry sauce and whipped cream. Garnish with fresh berries and mint sprigs, if desired.
Serve immediately.
