Turkey Potpie with Cheddar Biscuit Crust

Ingredients

• 10 ounce peas frozen thawed
• 2 teaspoons double-acting baking powder 
• 1 teaspoon baking soda 
• 0.5 teaspoon pepper black
• 12 pound skin-on turkey breast half 
• 1.3 cups buttermilk well-shaken
• 2 large carrots cut into 1/2-inch pieces
• 2 rib celery stalks cut into 1/2-inch pieces
• 1 cup sharp cheddar cheese extra-sharp grated
• 0.3 cup flour all-purpose
• 2 cups flour all-purpose
• 0.5 pound mushrooms trimmed quartered
• 1 medium onion coarsely chopped
• 0.3 cup parmesan grated
• 1 large parsnips cored peeled cut into 1/2-inch pieces ()
• 0.5 teaspoon salt 
• 1 teaspoon thyme leaves chopped
• 4 cups turkey meat cut into 1/2-inch pieces
• 0.8 stick butter unsalted cold cut into 1/2-inch pieces
• 3 tablespoons butter unsalted
• 10 cups water 

Equipment

• bowl
• oven
• pot
• sieve
• blender
• baking pan

Directions

1. Separate parts of carcass and put, along with skin, in an 8-quart pot. Cover bones with water and simmer until liquid is reduced by one third, about 1 1/2 hours. Strain through a fine-mesh sieve into a large bowl. Set aside 3 1/2 cups stock (reserve remainder for another use).
2. Cook onion, carrots, celery, parsnip, and thyme in butter with 1/2 teaspoon salt and 1/4 teaspoon pepper in a 12-inch-wide shallow pot (3- to 4-quart), covered, over medium heat, stirring occasionally, until vegetables are almost tender, 10 to 12 minutes.
3. Add mushrooms and cook, uncovered, stirring, until tender, 5 to 7 minutes.
4. Sprinkle with flour and cook, stirring constantly, 2 minutes. Stir in stock (3 1/2 cups), scraping up any brown bits, and bring to a boil, stirring, then simmer until slightly thickened, about 3 minutes. Stir in turkey, peas, and salt and pepper to taste. Reheat over low heat just before topping with biscuit crust.
5. Preheat oven to 400F with rack in middle.
6. Sift together flour, baking powder, baking soda, salt, and pepper into a medium bowl.
7. Add cheeses and toss to coat. Blend in butter with a pastry blender or your fingertips until mixture resembles coarse meal.
8. Add buttermilk and stir just until a dough forms. Drop biscuit dough onto filling in 8 large mounds, leaving spaces between biscuits.
9. Bake until biscuits are puffed and golden brown and filling is bubbling, 35 to 40 minutes.
10. Let stand 10 minutes before serving.
11. Cooks' notes: If using carcass and meat from a brined turkey, filling may need little or no salt. Filling can be made in a 12-inch skillet and transferred to a 13- by 9-inch baking pan before topping with biscuit dough. You can substitute another turkey stock or reduced-sodium chicken broth for the stock in this recipe. You can also make stock using leftover cooked chicken in place of turkey.