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1
Bring the stock to a simmer in a medium saucepan.
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2
Add the thyme leaves, then poach the chicken breasts until just firm and cooked through, 10-12 minutes.
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3
With a pair of kitchen tongs, transfer the poached chicken to a plate and let cool.
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4
Tip the boiling onions into the stock and simmer for 5 minutes.
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5
Add the mushrooms and continue simmering until both the onions and mushrooms are tender, 4-5 minutes longer.
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6
Using a slotted spoon, transfer the onions and mushrooms to a large bowl.
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7
Increase the heat under the stock and boil until it has reduced to 1 1/4 cups.
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8
Meanwhile, cut the chicken into bite-size pieces and add to the onions and mushrooms.
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9
When the stock has reduced, pour it into a pitcher.
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10
Return the pan to the heat.
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11
Melt the butter in the pan and stir in the flour.
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12
Keep stirring over medium heat for 3-4 minutes.
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13
Gradually pour in the hot stock, stirring until smooth.
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14
Simmer until thickened, 5-10 minutes, then stir in the cream and bring back to a simmer.
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15
Season to taste with salt and pepper.
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16
The sauce should be thick and creamy.
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17
Pour it over the chicken and vegetables and mix well.
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18
Let cool completely.
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19
Preheat the oven to 400F.
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20
Divide the pastry into two portions: two-thirds and one-third.
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21
Roll out the larger portion on a lightly floured surface to a circle large enough to line a pie dish or pan, or a casserole, that is 9-10 inches in diameter and 1 1/2-2 inches deep.
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22
Line the dish and trim off the excess pastry.
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23
For a good-looking result, put a pie bird or pie funnel in the middle of the dish.
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24
Spread the filling evenly in the dish.
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25
Roll out the remaining pastry to form a lid for the pie and cut a cross in the middle to fit around the pie bird.
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26
(If you're not using a pie bird, cut a small cross anyway, to serve as a steam vent.)
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27
Brush the pastry rim with the egg wash, then drape the pastry lid over the pie and press down around the rim to seal.
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28
Use a sharp knife to cut off any excess pastry around the rim, then crimp the edges.
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29
If you wish, decorate the pie with pastry leaves made with the trimmings.
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30
Brush the pie top and trimmings with the egg wash and sprinkle the top with coarse sea salt.
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31
Bake until the pastry is golden brown and the sauce is bubbling from the steam hole, about 35 minutes.
