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1
Using a mandoline or sharp knife, cut the potatoes into slices about 1/8 of an inch thick.
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2
Place them in a large bowl of cold water and soak for 30 minutes, changing the water twice.
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3
Drain and pat them dry.
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4
Arrange two oven racks so that one is in the bottom third of the oven and the other is in the middle.
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5
Preheat the oven to 425 degrees.
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6
Butter a shallow 10-inch gratin dish or glass baking dish.
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7
Thinly slice the shiitake caps.
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8
Cut the Brie into small cubes.
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9
Layer a third of the potato slices in the dish.
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10
Lay half the shiitakes and half the cheese evenly over the top.
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11
Liberally season with salt and pepper.
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12
Add another third of the potatoes and top with remaining shiitakes and Brie.
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13
Season with salt and pepper.
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14
Arrange remaining potato slices on top.
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15
Combine the cream, garlic and thyme and pour over the potatoes, pushing down so that all the liquid is absorbed and spreading the garlic and thyme evenly.
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16
Cover the pan tightly with foil and bake on the middle rack for 30 minutes.
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17
Combine the Parmesan and bread crumbs and season the mixture with more salt and pepper.
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18
Remove the foil from the pan and sprinkle the crumbs over the potatoes.
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19
Place the pan on a rack in the bottom third of the oven and bake 30 to 40 minutes longer, until the potatoes are very tender and the top and bottom are crusty and dark brown.
