Chicken Potpie

Combine chicken, chicken stock, yellow onion, bay leaves, peppercorns, 3 thyme sprigs, and celery in a stockpot, and add enough water just to cover the chicken. Bring stock to a boil, reduce heat, and simmer, uncovered, for 1 hour.

Pick enough thyme leaves to make 3 tablespoons. Combine 1 cup flour, 1/4 teaspoon salt, and 1 tablespoon thyme leaves in the bowl of a food processor, and set remaining 2 tablespoons thyme aside. Add 10 tablespoons chilled butter cut into small pieces, and pulse until mixture resembles coarse meal. While the food processor is running, add 3 tablespoons ice water and 1 egg yolk, and process until the dough holds together. Turn dough onto plastic wrap, flatten into circle, and wrap well; refrigerate at least 1 hour.

Drain chicken, and reserve the stock. Remove the skin from the chicken, and remove all the chicken from the carcass. Shred the chicken into bite-size strips, and set aside. Strain the stock, and set aside 2 cups. Reserve the remaining stock for another use.

Preheat oven to 375 degrees. Melt the remaining 5 tablespoons of butter in a large saute pan over medium-high heat. Add red potatoes and pearl onions, and cook, stirring occasionally, until the potatoes begin to turn golden, 4 to 5 minutes. Add leeks, carrots, and mushrooms, and cook 4 to 5 minutes more. Add the remaining 5 tablespoons flour, and cook, stirring, for 1 minute. Stir in reserved chicken stock and milk, and bring to a simmer. Cook until thick and bubbly, stirring constantly, 2 to 3 minutes. Add reserved chicken pieces, parsley, remaining 2 tablespoons thyme, lemon zest, remaining 2 teaspoons salt, and the ground pepper; transfer to a large casserole dish. Set aside.

Roll out the dough until it is 1/4 inch thick, and transfer to a baking sheet. Allow to chill 15 minutes. Make an egg wash: In a small bowl, whisk together the remaining yolk and heavy cream. Working quickly, place the dough over the top of the chicken mixture, and tuck extra dough around the edges. Cut slits on top to allow steam to escape. Brush with the egg wash, and place on a baking sheet. Bake until crust is golden, 35 to 40 minutes.