Whisk the egg whites. Add lemon juice, icing sugar, liqueur and peppermint oil. Mix briefly with the whisk of the hand mixer. Pour the mixture into a piping bag with a perforated spout and squirt generously into the oval chocolate moulds.
Leave to dry overnight at room temperature. Melt the couverture, put it in a freezer bag and cut off a small corner. Spray the chocolate coating onto the chocolates in a spiral. Place in a cool place.