Mix meat, chopped mushrooms, chopped water chestnuts, chopped onions, dressing and egg yolk until well blended. Spoon evenly onto won ton wrappers, adding about 1 tsp. of the meat mixture to each wrapper.
Beat egg white lightly. Brush onto edges of each wrapper; fold in half to form triangle. Press edges together to seal. Bring opposite corners of long edge of each triangle together, overlapping corners; brush with egg white to seal.
Combine chicken broth, mushrooms, water chestnuts and onions in large saucepan. Bring just to boil on medium heat. Carefully add won tons; simmer 4 min. or until filling is cooked through, stirring occasionally. Serve immediately.
Tip: Dont overfill the wontons or they will burst in the soup.