Beat the egg whites with a pinch of salt until a light foam forms. Add the sugar for the meringue and beat on high speed for 5 minutes.
Add the lemon juice for the meringue and continue beating for another 5-7 minutes. Add the cornstarch for the meringue and mix for 1 minute until you have a stable, stiff peak meringue.
Spread the meringue into a 1.5-2 cm thick rectangle on a parchment-lined baking sheet. Sprinkle the crushed peanuts evenly over the top.
Bake in a preheated oven at 160°C (320°F) for 30 minutes. Once baked, remove from the oven and let it cool.
While the meringue bakes, prepare the berry sauce. In a saucepan, combine the frozen blueberries, sugar for the sauce, lemon juice for the sauce, and cornstarch for the sauce. Cook over medium heat, stirring constantly, until the mixture thickens, about 5-7 minutes. Let it cool.
For the filling, whip the cold heavy cream with the powdered sugar until it begins to thicken. Add the mascarpone cheese and beat until thick and stable.
Flip the cooled meringue onto a new piece of parchment paper and carefully peel off the parchment it was baked on.
Spread the mascarpone cream evenly over the meringue, reserving a little for decoration on top. Pipe or spoon the cooled berry sauce in lines over the cream.
Carefully roll the meringue into a tight log using the bottom parchment paper to assist you.
Decorate the top of the roll with the reserved cream, fresh raspberries, and fresh blackberries. Slice and serve.
Cook it hands-free.
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