Heat the berries, caster sugar and one tablespoon of the lemon juice in a small pan, stirring well to combine, for 3-4 minutes, or until the sugar has dissolved and the berries have broken down. Set aside to cool.
Place the biscuits into a freezer bag and crush using a rolling pin until they resemble breadcrumbs.
Melt the butter in a small saucepan over a low to medium heat.
Mix together the crushed biscuits and melted butter in a bowl until well combined. Spoon the mixture into the bottom of two large serving glasses.
Place the mascarpone into a large mixing bowl and fold in the lemon zest, remaining lemon juice and icing sugar.
Spoon the mascarpone into the glasses, then spoon over the berries and juice. Serve immediately.