Pat the calves' liver dry and season. Lay a sage leaf on each slice and cover with a slice of Parma ham. Secure in place with a couple of cocktail sticks. Heat a griddle pan and lightly grill the liver for 2-3 minutes each side. Keep warm.
To make the dressing place the mustard seeds in a dry frying pan and heat until the seeds start to ‘pop’. Remove from the heat and when the seeds have stopped ‘popping’ tip into a bowl and add the olive oil, garlic and lemon juice whisking until all the ingredients and blended well together. Add the celeriac, apple and watercress to the dressing and toss well.
To serve, place a slice of liver on each plate and pile high with salad.