Cut chicken into 1" cubes; pat dry In large heavy saucepan, melt butter with oil over medium/high heat until butter foams and subsides. Add chicken , cooking for 7-10 minutes or until golden. Remove with slotted spoon and set aside. In same pan, cook onion, celery and carrot over medium/high heat for 5-7 minutes or until softened. Stir in salt, thyme, pepper and bay leaf. Pour in 2 cups of the stock; bring to boil. Blend flour with remaining stock; stir flour mixture into pan. Cook over medium heat until sauce thickens and smoothes, stirring constantly. Simmer for 8-10 minutes longer or until vegetables are tender-crisp. Remove from heat; remove bay leaf. Add peas, parsley and chicken. Spoon equal portions into four 2 cup oven proof bowls. Roll pastry to 1/4" thickness. Cut out circles about 1" wider than tops of bowls. Stir together egg yolk with water; brush lightly over outside rims of bowls. Lay pastry over bowls, pressing to outside. Brush remaining egg yolk mixture over pastry. Slit an X in middle. I have also done it as one large pot pie and it works well too. On occasion, I have made double the filling ahead and frozen it to have on hand for a fast meal almost prepared when the family desends on me. Bake in 400F oven for 30-35 minutes or until pastry is golden brown and filing is bubbly.