Preheat your oven to 300°F (150°C). Line a baking sheet with parchment paper and draw a 10-inch circle as a guide for shaping the meringue.

In a large bowl, whip room temperature egg whites and cream of tartar until soft peaks form, about 3-4 minutes. Gradually add sugar, continuing to whip until stiff, glossy peaks form, about 5-7 minutes.

Gently fold in cornstarch and lemon juice with a spatula, being careful not to deflate the mixture.

Spoon the meringue mixture onto the prepared parchment paper, using the drawn circle as a guide, and create a slight well in the center.

Bake the meringue for 45-55 minutes until dry and lightly golden. Turn off the oven and let it cool inside for about 1 hour without opening the door.

In a clean bowl, whip heavy cream until soft peaks form. Gradually add remaining sugar, continue beating until glossy and holds shape.

Toss blackberries and sliced strawberries with a sprinkle of sugar; let sit for about 10 minutes to enhance their sweetness.

Transfer the cooled meringue to a serving plate, add whipped cream to the center, and top with berries.