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1
Butter a 13-by-9-by-2-inch baking dish.
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2
In a saucepan, combine the turkey stock, potatoes, thyme and a large pinch of salt and bring to a boil over high heat.
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3
Reduce the heat to low, cover and simmer until the potatoes are tender, about 10 minutes.
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4
Meanwhile, in a large skillet, melt 1 tablespoon of the butter in 1 tablespoon of the oil.
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5
Add the white mushrooms, season with salt and pepper and saute over moderately high heat until golden, about 5 minutes.
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6
Transfer to a bowl.
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7
Repeat the process with 1 more tablespoon each of the butter and oil and the shiitakes.
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8
Add to the white mushrooms.
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9
Melt the remaining 1 tablespoon of butter in the remaining 1/2 tablespoon of oil.
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10
Add the onion and prosciutto and saute over moderately high heat until the onion softens, about 5 minutes.
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11
Remove from the heat and add the mushrooms.
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12
Strain the turkey stock into the skillet.
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13
Add the flour mixture and bring to a boil over moderate heat, stirring constantly.
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14
Add the turkey, potatoes and parsley and season with salt and pepper.
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15
Spread the mixture in the baking dish and let cool.
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16
Preheat the oven to 400.
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17
If necessary, roll out the puff pastry on a lightly floured surface to a 14-by-10-inch rectangle.
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18
Moisten the rim of the baking dish with water and cover the filling with the dough, pressing it firmly against the baking dish rim; tuck the edges under.
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19
Make a few slits in the pastry and brush with the beaten egg.
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20
Bake for 45 minutes to 1 hour, or until the top is golden.
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21
Let the potpie stand for 10 minutes before serving.
