Chicken Pot Pie With Biscuit Crust

Name: Chicken Pot Pie With Biscuit Crust
Ingredients: 4 cups chicken broth; 3 carrots, cut crosswise into ¼-inch slices; ¾ pound red potatoes, quartered lengthwise and cut crosswise into ½-inch pieces; 2 large ribs of celery, cut crosswise into ½-inch pieces; 2½ cups cubed or shredded chicken (the meat from a 3-pound cooked chicken); 1 onion, chopped; ¾ stick (6 Tbsp.) unsalted butter; 6 Tbsp. all-purpose flour; ¼ tsp. dried thyme, crumbled; ¼ tsp. freshly grated nutmeg, or to taste; ½ cup finely chopped fresh parsley leaves; Kosher salt and freshly ground black pepper; 1⅓ cups all-purpose flour; 1½ tsp. double-acting baking powder; ½ tsp. baking soda; ½ tsp. kosher salt; 2 Tbsp. cold unsalted butter, cut into bits; 2 Tbsp. cold vegetable shortening, cut into bits; ⅓ cup grated sharp cheddar; 1 large egg; About ⅓ cup buttermilk; An egg wash made by beating 1 large egg yolk with 1 Tbsp. milk