Preheat oven to 400°. Spray 2-quart baking dish with no-stick cooking spray; set aside.
Heat oil in large saucepan over medium heat and brown bacon, stirring occasionally. Stir in mushrooms and cook, stirring occasionally, until mushrooms start to soften, about 5 minutes. Stir in leek and chicken. Cook covered over medium-low heat, stirring occasionally, 5 minutes.
Stir in boiling water, heavy cream, peas, Knorr® Homestyle Stock - Chicken and Hellmann's® or Best Foods® Real Mayonnaise. Reduce heat to low and simmer uncovered, stirring occasionally, until chicken is thoroughly cooked and liquid starts to thicken, about 7 minutes. Turn into prepared baking dish and let cool 5 minutes.
Cover with pastry sheet, pressing edges to seal. Brush with beaten egg. Bake until pastry is golden brown, about 35 minutes. Serve, if desired, with garlic mashed potatoes, roasted vegetables and a salad.