1. Preheat oven to 225°. Whisk together sugar and cornstarch. Beat egg whites at medium-high speed with a heavy-duty electric stand mixer 1 minute; add cream of tartar and salt, beating until blended. Gradually add sugar mixture, 1 Tbsp. at a time, beating at medium-high speed until mixture is glossy, stiff peaks form, and sugar dissolves. (Do not overbeat.) Beat in vanilla. Gently spread mixture into a 7-inch round on a parchment paper-lined baking sheet, making an indentation in center of meringue to hold filling.
2. Bake at 225° for 1 hour and 30 minutes or until pale golden and outside has formed a crust. Turn oven off; let meringue stand in oven, with door closed and light on, 12 hours.
3. Meanwhile, whisk together lemon curd and sour cream until smooth. Cover and chill.
4. Spoon lemon mixture into center of meringue, and top with berries. (Center of meringue may fall once the lemon mixture and berries have been added.) Garnish, if desired.