Begin by lining a baking sheet with parchment paper, using a pencil to draw a 10-inch circle on it as a guide.

Set your oven to 300°F (150°C) and let it preheat fully.

In a clean bowl, whip the egg whites with cream of tartar until soft peaks form, about 2 minutes. Gradually add sugar, one tablespoon at a time, beating on high until stiff, glossy peaks form (5-7 minutes).

Gently fold in cornstarch and lemon juice with a spatula.

Spoon meringue into the parchment circle, shaping with a slight well in the center.

Bake for 45-55 minutes until the outside is crisp. Turn off oven, leaving the meringue inside for an additional hour to cool.

While cooling, rinse blackberries and strawberries. Toss with sugar and let them sit for 10 minutes.

In another bowl, whip heavy cream until thick, adding remaining sugar until soft peaks form.

Once the meringue is cooled, transfer it to a serving plate, spread whipped cream on top, and add the marinated berries.