1. Rinse chicken and place in a saucepan with broth, bay leaf, and 2 cups water. Bring to a boil, reduce heat and simmer until chicken is almost cooked through, about 10 minutes. Remove from heat, cover and let stand until chicken is cooked through, about 10 minutes. Transfer chicken to a cutting board, reserving 3 cups cooking liquid in saucepan. When chicken is cool enough to handle, pull apart into bite-size pieces and set aside.
2. Heat oil in a large heavy nonstick skillet over moderately high heat. Add celery, onion, mushroom, carrot, potato, thyme, and tarragon. Cook, stirring occasionally, until vegetables are softened, about 4 minutes. Sprinkle flour over vegetables and cook, stirring, 1 minute. Add reserved cooking liquid in a slow stream, stirring constantly, and simmer until slightly thickened, about 5 minutes. Transfer to a large heat proof measuring cup or bowl and stir in chicken, parsley, nutmeg, and juice, for filling. Let cool.
3. Divide filling between six 2-cup ovenproof bowls and place in a shallow baking pan.
4. Preheat oven to 350 degree F and arrange rack in middle of oven. Unfold 1 piecrust on a lightly floured surface. Using a floured rolling pin, roll out into a 14-inch round, then cut out three 5-1/2-inch rounds, using a plate as a guide. Repeat with remaining piecrust. Brush edges of each round with egg and place, egg side down, over filling in bowls, pressing piecrust edge on rim to seal. Brush tops of crusts with egg and bake until golden brown, about 25 minutes. Transfer pies to rack and cool 15 minutes before serving (bowls will be hot).To freeze::
5. Once cooled, tightly wrap each pie in aluminum foil and freeze up to 1 week. Unwrap and reheat in a 350 degree F oven until hot, about 40 minutes. Makes 6 servings.