Preheat oven to 400°F. Let piecrusts stand at room temperature 15 minutes. Unroll 2 piecrusts onto a lightly floured surface. Cut 3 (5-inch) circles from each crust. Gently roll each circle into an 8-inch round. Coat 6 (12-ounce) ramekins with cooking spray. Gently fit 1 piecrust round into each ramekin, lining the bottom and sides. Gently crimp dough over the top edge of ramekin to secure it. Prick dough with a fork along bottom and sides. Place ramekins on a rimmed baking sheet, and bake in preheated oven until lightly browned, 13 to 16 minutes.

Melt 2 tablespoons of the butter in a large skillet over medium-high. Add chicken; cook until done, 5 to 6 minutes per side. Remove from pan; let stand 10 minutes. Shred the chicken.

Add remaining 6 tablespoons butter to skillet. Add mushrooms; cook, stirring occasionally, 5 minutes. Add potato, onion, carrot, thyme, garlic, and oregano to skillet; cook, stirring occasionally, until onions are tender and lightly browned, 7 to 8 minutes.

Whisk together flour and 1/2 cup of the stock in a small bowl, and add to skillet. Stir in salt, black pepper, crushed red pepper, and remaining 2 cups stock; bring to a boil. Reduce heat to medium, and simmer 5 minutes, stirring occasionally. Stir in peas, cream, and shredded chicken. Transfer mixture to a bowl, and cool 10 minutes.

Stir together egg yolk and water in a small bowl. Unroll remaining 2 piecrusts onto lightly floured surface. Cut 3 (5-inch) circles from each piecrust. Gently roll each circle into a 5 1/2-inch round. Fill each ramekin with about 3/4 cup chicken mixture, pressing down to level mixture. Top each ramekin with 1 (5 1/2-inch) dough round, pressing to seal to bottom crust. Brush tops with egg mixture, and, if desired, top with leaf cutouts or rounds from extra dough. Cut small slits in tops of dough to vent steam. Bake at 400°F until golden brown, 22 to 25 minutes.