Preheat oven to 350°. In a large skillet, heat 2 tablespoons oil over medium-high heat. Add leeks and mushrooms; cook and stir 3-4 minutes or until vegetables are tender. Stir in flour until blended; gradually stir in stock and milk. Bring to a boil, stirring constantly; cook and stir 3-4 minutes or until thickened. Stir in chicken, 3/4 cup cheese, thyme, salt and pepper., Brush a 9-in. pie plate with some of the remaining oil. Place one sheet of phyllo dough into prepared pie plate, allowing ends to extend over edges of dish; brush with oil. (Keep remaining phyllo covered with plastic wrap and a damp towel to prevent it from drying out.) Layer with seven additional phyllo sheets, brushing each layer with oil and rotating sheets to cover the pie plate. Transfer chicken mixture to crust., Gently fold in ends of phyllo over filling, leaving an opening in the center. Crumble the remaining phyllo sheets over filling; sprinkle with remaining cheese. Brush edges with remaining oil. Bake 30-35 minutes or until golden brown.