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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).
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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. Add mushrooms and cook, uncovered, stirring, until tender, 5 to 7 minutes.
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3
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.
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4
Preheat oven to 400u00b0F with rack in middle.
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5
Sift together flour, baking powder, baking soda, salt, and pepper into a medium bowl. Add cheeses and toss to coat. Blend in butter with a pastry blender or your fingertips until mixture resembles coarse meal. Add buttermilk and stir just until a dough forms. Drop biscuit dough onto filling in 8 large mounds, leaving spaces between biscuits.
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6
Bake until biscuits are puffed and golden brown and filling is bubbling, 35 to 40 minutes. Let stand 10 minutes before serving.
