How to Make It
Preheat oven to 400°F. Place chicken, stock, and 1/2 teaspoon of the salt in a large saucepan; bring to a boil over high. Reduce heat to medium-low; cover and cook until chicken is cooked through, about 15 minutes. Remove from heat, and let stand about 20 minutes. Remove chicken from stock, reserving 2 1/2 cups of the stock. Coarsely shred chicken.
Meanwhile, place puff pastry sheet on a baking sheet lined with parchment paper. Cut pastry sheet into 4 squares; separate squares. Brush squares lightly with egg; top each square with 3 parsley leaves, pressing gently to adhere. Sprinkle with pepper. Bake on oven rack in bottom third of preheated oven until dough is puffed and golden brown, 12 to 14 minutes.
Cook peas according to package directions; keep warm.
Cook bacon in a large skillet over medium-high until crisp, about 6 minutes. Remove bacon to a paper towel-lined plate, reserving drippings in skillet; crumble bacon. Add butter to hot drippings in skillet, and cook over medium until butter melts, about 1 minute. Add onion and celery; cook, stirring often, until onion is tender and celery is tender-crisp, about 8 minutes. Add flour, and cook, stirring constantly, about 1 minute. Stir in cream and reserved 2 1/2 cups stock; bring to a simmer, stirring often. Stir in peas, cheese, chicken, bacon, and remaining 1/2 teaspoon salt; reduce heat to medium-low, and cook until mixture is thickened and thoroughly heated, about 10 minutes.
Divide mixture among 4 shallow bowls, and top each with a puff pastry square. Serve immediately.