Place the chocolate in a thin-sided bowl and suspend over a gently simmering bain-marie, ensuring that the bowl does not touch the water. Melt the chocolate and stir until smooth. Mix the biscuits, butter and cardamom, stir in the chocolate, transfer to the prepared tin, press down firmly, chill for approx. 15 mins. Arrange the whole strawberries over the base and the halved strawberries around the tin frame.
Puree the strawberries, icing sugar and lemon juice, pass through a sieve into a bowl. Dissolve the gelatine in the water, mix together with 2 tbsp of the mixture and immediately stir into the remaining mixture. Beat the egg white and salt until stiff, carefully fold into the strawberry mixture along with the whipped cream, transfer to the prepared tin, place the strawberries in the mixture, cover and chill for approx. 3 hrs.
Remove the mousse from the tin, cut into 4-6 slices, decorate with the berries.