Meringue: Preheat the oven to 200°F with the rack on the bottom third of the oven. Line a baking pan with parchment and trace a 7-inch circle. Flip the parchment over so the circle is on the other side.
In a stand mixer with the whisk attachment, beat the egg whites and salt on medium-low speed until fluffy, about 5-6 minutes. Turn the mixer to medium speed, gradually add the sugar a tablespoon at a time with 10 seconds in between each addition. Scrape down the sides of the bowl with a rubber spatula. Increase the speed to medium-high; beat until egg whites are thick, glossy, and stiff peaks form, about 5-7 minutes.
Remove the bowl from the mixer and sift the cornstarch onto the beaten egg whites. Add the vanilla and white wine vinegar; fold in lightly with a rubber spatula. Scoop the meringue into the center of the traced circle on the parchment paper. Spread it into a circle. Bake until the meringue exterior is dry and crisp, 2 to 2½ hours. (2 hours for a more marshmallow-like center). Turn the oven off and let the meringue completely cool, about 1 hour or up to overnight.
Red Berry Sauce: Puree half of the strawberries and raspberries in a food processor. Strain the puree through a sieve and discard the solids.
In a small saucepan over medium heat, cook the berry juices until reduced to 1 cup, about 7-9 minutes. Remove from heat and stir in lemon juice; cool completely. Combine ¼ of the red berry sauce with the remaining half of fresh berries. Gently fold to coat the berries.
Whipped Cream Topping : In a medium bowl with a hand mixer, beat the cream and crème fraiche (or sour cream) to medium peaks. Add the powdered sugar and vanilla extract; beat until just combined.
Gently fold in 4-5 tablespoons Red Berry Sauce into the whipped cream leaving streaks of sauce. Spoon whipped cream on top of meringue, then the fresh berries in berry sauce.
Garnish with fresh mint, fresh basil, or lemon zest; optional. Serve with the remaining Red Berry Sauce on the side.