Place the egg whites, powdered sugar, cornflour and lemon juice in a bowl. Beat the egg whites with an electric mixer until the meringue is firm and smooth. Fill a piping bag fitted with a plain tip with the meringue. Poach in a half-sphere mould on a baking sheet lined with baking paper.
Bake the meringue in an oven preheated to 95°C for approx. 2 hours. Turn off the oven and leave the meringue in the turned-off oven for a further 30 minutes.
Melt a little white chocolate in the microwave and brush the inside to protect the meringue from the moisture of the strawberries.
Whip the cold cream with the mascarpone using an electric mixer. Gradually add the powdered sugar. Fill a piping bag fitted with a plain tip with this whipped cream.
Assembly
Sand the underside of the meringue with a rasp to ensure it is stable. Turn over the cold meringue. Garnish the center with diced, unsweetened strawberries, then poach the chantilly mascarpone.
Top with diced strawberries, cover entirely with whipped cream and chill until ready to eat.