PREPARATION:

1. To make chicken stock, place chicken, sliced onion, 1/3 of carrots and 1/3 of celery in Dutch oven. Add enough cold water to cover. Cover and bring to a boil over medium heat. Reduce heat to low. Simmer 5 to 7 minutes or until chicken is no longer pink in center.
2. Remove chicken; cool. Strain stock through large sieve lined with several layers of dampened cheesecloth; discard vegetables. Refrigerate stock; skim off any fat that forms on top. Measure 2 cups stock.
3. When chicken is cool enough to handle, remove skin and bones; discard. Cut chicken into bite-size pieces.
4. Place remaining carrots, celery and enough water to cover in medium saucepan. Cover; bring to a boil. Reduce heat to medium-low; simmer 8 minutes or until vegetables are crisp-tender. Set aside.
5. Preheat oven to 400°F. Roll puff pastry out on lightly floured surface to 12X8-inch rectangle. Place on ungreased baking sheet; bake 15 minutes. Set aside.
6. Melt butter in large saucepan over medium-high heat. Add chopped onion and mushrooms; cook and stir 5 minutes or until tender. Stir flour, basil, salt and pepper. Slowly pour in reserved chicken stock and milk. Cook until mixture begins to boil. Cook 1 minute longer, stirring constantly.
7. Stir in reserved chicken, peas, carrots and celery. Cook until heated through. Pour mixture into 12X8-inch baking dish; top with hot puff pastry. Bake 5 minutes until puff pastry is brown. Garnish as desired.