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1
Preheat the oven to 350.
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2
In a pot of boiling salted water, blanch the haricots verts until crisp-tender, about 2 minutes.
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3
Drain and cool under running water.
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4
Pat dry.
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5
In a very large, deep skillet, melt 4 tablespoons of the butter.
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6
Add the mushrooms, season with salt and pepper and cook over moderately high heat, stirring frequently, until lightly browned, 6 to 7 minutes.
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7
Add the onion and garlic and cook over low heat, stirring occasionally, until softened, about 5 minutes.
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8
Add the corn and cook, stirring occasionally, until the mixture is dry and the corn is just beginning to brown, about 5 minutes.
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9
Add the haricots verts and keep warm.
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10
In a large saucepan, melt the remaining 6 tablespoons of butter.
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11
Add the flour and whisk over moderate heat until fragrant and lightly browned, about 4 minutes.
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12
Whisk in the milk until smooth and bring to a boil.
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13
Reduce the heat to moderately low and simmer, whisking constantly, until very thick, about 5 minutes.
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14
Remove from the heat.
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15
Stir in the Gruyere and season with salt, pepper and nutmeg.
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16
Pour the sauce over the vegetables and stir well.
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17
Pour the mixture into a 4-quart baking dish.
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18
In a small bowl, combine the panko with the Parmigiano and sprinkle over the vegetables.
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19
Set the baking dish on a rimmed baking sheet and bake in the center of the oven for 30 minutes, until bubbling.
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20
Turn on the broiler and broil the casserole for 3 to 4 minutes, until the top is golden and crisp.
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21
Let the casserole rest for 10 minutes before serving.
