Preheat the oven to the lowest temperature and put the shelf at the bottom of the oven. Line two baking trays with greaseproof paper and draw 12 x 5cm/2in circles onto the paper, leaving at least 2cm/Â¾in between them.
For the meringue, whisk the egg whites and lemon juice in a bowl until soft peaks form. Gradually whisk in the sugar until fully incorporated, then continue to whisk until stiff peaks form when the whisk is removed.
Spoon the meringue into a piping bag fitted with a small star nozzle and pipe nests onto the baking trays using the circles to guide you.
Bake for 1Â½-2 hours or until the meringues lift off the greaseproof paper easily; they should not be coloured. Set aside to cool on a wire rack.
Meanwhile, for the mousse, whisk the egg whites until stiff peaks form when the whisk is removed. In a separate bowl, whisk the double cream to stiff peaks and fold in the icing sugar.
Blend the strawberries in a food processor and pass through a fine sieve.
Squeeze out the gelatine leaves and place in a small pan with four tablespoons of the strawberry purÃ©e. Heat gently, stirring until the gelatine has dissolved. Leave to cool a little then mix in the remaining strawberry purÃ©e and fold into the double cream. Fold this mixture into the egg whites, then cover with cling film and chill in the fridge.
To assemble, spoon the mousse in a piping bag fitted with a small plain nozzle. Pipe the mousse into the meringue nests and top with a halved strawberry, cut-side up.