1.Mix Flour And Margarine Together In A Saucepan Over Medium Heat, Stirring Until The Flour And Margarine Form A Paste. 2.Whisk Cream Into Flour Mixture About 1/2 Cup At A Time Until Smoothly Combined; Whisk In Chicken Broth And Parsley. Bring Sauce To A Simmer, Whisking Constantly Until Thickened. 3.Whisk Egg Yolks And Lemon Juice Together In A Bowl. 4.Whisk Egg Yolk Mixture Slowly Into Cream Mixture, About 1 Tablespoon At A Time, Until The Sauce Barely Comes To A Simmer And Coats The Back Of A Spoon. Do Not Boil. 5.If Thicker Sauce Is Desired, Whisk Cornstarch And 1/4 Cup Of Sauce In A Small Bowl; Stir Cornstarch Mixture Back Into Sauce Until Thickened, About 2 Minutes. 6.Stir Chicken Breast Pieces Into Sauce Until Chicken Is Heated Through.