Preheat the oven to 200°C. Roast the Yorkshire puddings for 4-5 minutes, or according to pack instructions, until crisp and golden brown. Cool.
Meanwhile, blend all the ingredients together for the pistou in a food processor until smooth.
Combine the cream cheese with the crème fraiche then stir in half of the pistou sauce.
Spoon teaspoons of cream cheese mixture into the bottom of each Yorkshire pudding, top with a little more pistou and then finish with a sprig of watercress and half a slice of Parma Ham.
Recipe courtesy of Parma Ham