Pour the egg whites, powdered sugar, cornstarch, and lemon juice into a mixing bowl. Beat the egg whites with an electric mixer until you obtain a firm, smooth meringue. Fill a piping bag fitted with a plain tip with the meringue. Pipe the meringue onto a baking sheet lined with parchment paper.
Bake at 90°C for 2 hours and 30 minutes, then leave in the oven for another 30 minutes with the oven turned off.
Using a kitchen grater, lightly sand the meringues so that they stand upright when turned over.
Melt a little white chocolate in the microwave and brush it over the surface to protect it from moisture.
Using an electric mixer, whip the very cold heavy cream with the mascarpone. Use heavy cream with at least 30% fat content (I often use Président Fleurette Extra cream). Gradually add the powdered sugar. Fill a piping bag fitted with a plain tip with this whipped cream.
Assembly
Pipe the whipped cream onto the meringues.
Then spoon the strawberry compote into the middle of the whipped cream.
Place the strawberry slices on top. I added some small mint leaves.