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1
Place an oven rack in the center of the oven and preheat to 400 degrees F. Line a baking sheet with parchment paper.
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2
For the filling: Heat the oil over medium-high heat in a large skillet.
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3
Cook the leeks until slightly softened, about 3 minutes.
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4
Add the carrots, mushrooms, potato, garlic, thyme, salt and pepper.
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5
Stir frequently until the vegetables are tender, about 12 minutes.
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6
For the sauce: Melt the butter over medium heat in a 1-quart saucepan.
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7
Add the flour and whisk until smooth, about 2 minutes.
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8
Add the milk, whisking constantly to prevent lumps.
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9
Simmer over medium heat, whisking constantly, until the sauce is thick and smooth, about 10 minutes.
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10
Remove the pan from the heat and stir in the cheese, salt, pepper and nutmeg.
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11
Combine the vegetable mixture, sauce and chicken in a medium bowl.
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12
Season with salt and pepper.
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13
Cool the filling completely, about 20 minutes.
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14
Roll out the pastry into a 10-by-10-inch square on a lightly floured surface.
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15
Transfer the pastry to the prepared baking sheet.
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16
Lightly brush the edges of the pastry with beaten egg using a pastry brush.
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17
Spoon the filling onto one side of the pastry.
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18
Fold the pastry over to encase the filling.
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19
Fold and crimp the edges of the pastry together to seal the filling.
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20
Brush the top of the pastry with beaten egg.
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21
Cut a 1/2-inch-long slit in the top of the pie using a paring knife.
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22
Bake until puffed and golden, 30 to 35 minutes.
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23
Cool for 15 minutes and slice into 4 pieces.
