Prepare the mousse: Place chicken in a food processor with egg yolk, whole egg and cream. Puree until smooth. Add parsley, salt and pepper and pulse until parsley is incorporated. Mousse should not be too uniformly green but speckled with parsley. Place a piece of plastic wrap on a work surface. Put 1/8 of the mousse mixture in a circle the diameter of the medallion on the plastic wrap. Place medallion on top of the mousse. Spread the top of the medallion with another 1/8 mousse mixture. Wrap a piece of prosciutto around the outside of the medallion. Pull the plastic wrap up around the whole medallion to tightly enclose. Repeat with remaining three veal medallions. Place medallions in a vegetable steamer over a pot of boiling water. Cover the steamer tightly and cook to desired doneness (about 20 minutes for medium).
Prepare the watercress: Bring a medium saucepan of water to a boil. Submerge the watercress into the water. Cook until tender about 2 minutes. Drain. Place into a food processor and puree until smooth adding cream, salt and pepper. Keep the mixture warm until ready to use.
Prepare carrots: Peel carrots and cut into strips 1/8 inch wide by 6 inch long. Bring a medium saucepan of water to a boil. Submerge carrots into the water. Cook until tender about 2 minutes. Drain. Put into a small saucepan with the butter. Season with salt and pepper. Hold warm until ready to use.
Make the sauce: Heat veal stock in a saucepan. Bring to a boil and add the heavy cream and butter. Keep warm until ready to use.
Assemble the dish: Remove veal from steamer. Unwrap and place in the middle of a serving plate. Place a spoonful of watercress puree on top of the veal fillet. Place a bunch of spaghetti of carrots on top of the watercress. Drizzle plate with sauce. Optional accompaniment: shredded potato pancake, Sauteed wild mushrooms, roasted garlic or roasted shallots.