Soak the gelatine in cold water for 5 minutes to soften.
Meanwhile, place the stock in a saucepan over medium-high heat and bring to the boil. Squeeze any excess water from the gelatine, then add the gelatine to the stock, stirring until dissolved. Set aside to cool.
Combine ham, cornichons, capers and parsley in a bowl, then season. Pack into four 1 cup (250ml) ramekins and pour over the cooled stock mixture. Cover and chill overnight to set.
The next day, briefly dip the base of each ramekin in hot water, then invert the jellied hams onto serving plates. Serve with sourdough and mustard.