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1
Heat the oven to 400F.
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2
Wrap the beets in aluminum foil and roast until theyre just barely cooked through, about 50 minutes (you should be able to poke a fork in about 1/2 inch with some resistance).
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3
Reduce the oven temperature to 375F.
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4
When the beets are at room temperature, trim off their ends, peel, and slice into 1/2-inch-thick rounds (you should yield about 6 slices).
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5
Lay the slices on a large surface and season both sides evenly with a total of about a scant 1/2 teaspoon salt and a pinch of pepper.
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6
Meanwhile, melt 3 tablespoons of butter in a 12-inch, heavy, nonstick frying pan over medium-high heat.
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7
When melted, add the shallots and saute until translucent and aromatic, about 3 minutes.
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8
Add the mushrooms, thyme, 1 teaspoon of salt, and a large pinch of pepper, and saute until the mushrooms are tender and all of the liquid has evaporated, about 13 minutes.
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9
Add 1/4 cup of wine and simmer until all of the liquid has evaporated, about another 7 minutes.
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10
Remove and discard the thyme sprigs, and set the mushrooms aside off the heat to cool to room temperature (you can freeze them for about 15 minutes to expedite this process).
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11
Meanwhile, mix the soft goat cheese with the 1/4 cup chives, the zest, 1/4 teaspoon salt, and a large pinch of pepper.
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12
Sprinkle about half of the flour on a large clean work surface.
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13
Unfold the puff pastry onto the surface and evenly and lightly flour both sides.
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14
Carefully stretch it a bit so it measures 12 1/2 x 12 1/2 inches square.
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15
Working about 5 inches down the center of the dough rectangle (leave about 3 inches each at the top and bottom and roughly 4 inches each at both sides), evenly arrange half of the mushroom mixture, then the beet slices, then the cheese mixture, and then the remaining mushroom mixture.
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16
Fold both ends of the dough over the edges of the vegetables to cover.
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17
Then fold both sides of the dough toward the center to enclose the filling completely; shape into an even rectangle.
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18
Carefully transfer the filled pastry to a parchment paperlined baking sheet with sides.
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19
Brush the top and sides of the pastry with the egg wash. Refrigerate for 45 minutes to 2 hours.
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20
Bake in the center of the 375F oven until the pastry is golden brown, puffed, and completely firm when poked, about 50 minutes.
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21
Let sit until youve made the sauce.
