Heat a large saute pan over medium-high heat. Add the olive oil and heat. When the oil is hot, add the onions and cook for 1 minute. Add the mushrooms and chicken and cook until the mushrooms begin to give off their liquid and the chicken is no longer pink. Season with salt and pepper. Remove from the heat and add the port. Return to the heat and cook until the port is reduced by 1/4. Be careful as the mixture may ignite. Add the chicken stock and cook for 2 minutes. With a slotted spoon, remove the chicken and mushroom mixture and place in a bowl. Return the pan to the heat and reduce the liquid down until it is a syrupy consistency, about 5 minutes. Add the reduced liquid to the chicken mixture. Add the parsley and mix well. Cool.
Remove the bone from the thigh to the first joint of the leg. Reserve the bones and scraps for the steaming liquid. You can ask your butcher to do this for you. Season the legs, inside and out, with salt and pepper. Lay the legs down, skin-side down. Place 1/4 cup of the cooled filling inside each leg. Fold the meat over the filling and secure closed with a toothpick.
In the bottom of a steamer, combine the chicken stock, reserved bones, thyme, and olive oil and bring to a boil.
Place the steamer basket in the steamer. Add the chicken legs in 1 layer and cover. Steam until the chicken is cooked through, about 20 to 25 minutes.
Remove the chicken legs from the steamer and keep warm while you make the sauce. Place 1/2 cup of the steaming liquid in a blender. Add the watercress and blend well. With the blender running on low, add the butter, a little bit at a time. Season with salt, pepper, and lemon juice.
Slice the boneless part of the chicken leg into 3 slices. Place some sauce on a plate and top with the chicken leg.
Recipe adapted from Wolfgang Puck, "Modern French Cooking for the American Kitchen", Houghton Mifflin, 1981.