In a large bowl, whisk together the mascarpone, fromage frais and 100g lemon curd until smooth. Add the lemon juice and whisk briefly. Rinse the beaters.
In a separate bowl, whisk the egg white until stiff. Gently fold this into the lemon curd mixture, then divide between 6 small glasses, dishes or ramekins. Cover and chill for at least 30 minutes or up to 2 hours.
Meanwhile, mix the remaining curd with the liqueur to give a smooth sauce. Spoon a little over each mousse and serve with your favourite crunchy biscuits.