Start by preheating your oven to 300°F. Place egg whites in a clean mixing bowl and beat on medium speed with cream of tartar until soft peaks form. Gradually add sugar, a little at a time, until the mixture becomes glossy and stiff.
Line a baking sheet with parchment paper and invert a 6–8 inch circle as a guide. Spoon the meringue onto the paper, forming a shallow well in the center for the cream and berries. Bake for 45–50 minutes until the exterior is set and crisp. Turn off the oven and let the meringue cool inside for an hour to avoid cracks.
While the meringue cools, toss blackberries and sliced strawberries with a portion of the sugar and let them sit for 10 minutes. In a separate bowl, beat heavy cream with the remaining sugar until soft peaks form. This gives a light, fluffy texture that pairs perfectly with the meringue.
Carefully transfer the cooled meringue to a serving plate. Spoon whipped cream into the center, creating a small well. Top with the macerated berries, arranging them to make the dessert look vibrant and inviting.