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1
Preheat the oven to 375.
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2
Bring 4 inches of water to a boil in a large pot.
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3
Add the spinach in large handfuls and cook just until wilted, about 2 minutes.
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4
Drain the spinach and cool under running water.
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5
Drain the spinach again, pressing and squeezing out as much water as possible.
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6
Transfer the spinach to a cutting board and coarsely chop it.
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7
In a large, deep skillet, melt 4 tablespoons of the butter.
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8
Add the shiitake mushrooms, shallots and thyme and cook over moderate heat, stirring, until softened and just beginning to brown, about 8 minutes.
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9
Stir in the flour and cook, stirring, just until the pan looks dry, about 2 minutes.
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10
Gradually whisk in the milk and stir until smooth and creamy.
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11
Bring to a boil over moderate heat and cook, stirring, until very thick, 2 to 3 minutes.
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12
Stir in the spinach and half of the cheese and season with salt and pepper.
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13
Scrape the mixture into a shallow 1 1/2-quart baking dish in an even layer.
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14
In a small skillet, melt the remaining 2 tablespoons of butter.
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15
Remove from the heat and add the panko and the remaining cheese; season with salt and pepper.
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16
Scatter the crumbs over the spinach and bake for about 20 minutes, until the sauce is bubbling and the crumbs are golden.
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17
Let stand for 10 minutes before serving.
