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1
Heat the oven to 350 degrees F and arrange a rack in the middle.
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2
Place the panko in a large frying pan over medium heat and toast, stirring occasionally, until golden brown, about 10 minutes.
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3
Transfer to a small bowl and set aside.
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4
Wipe out the frying pan with paper towels, add 3 tablespoons of the butter, and melt over medium heat until foaming.
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5
Add the onion and cook, stirring occasionally, until just softened and starting to brown, about 15 minutes.
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6
Meanwhile, bring a large pot of heavily salted water to a boil over high heat.
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7
Add the green beans and cook until theyre bright green and crisp tender, about 3 minutes.
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8
(They will not be fully cooked.)
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9
Drain in a colander and rinse under cold water until the beans are cooled, then set aside.
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10
Reserve the large pot.
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11
When the onions are ready, remove the frying pan from the heat.
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12
Remove 1/4 cup of the onions and set aside.
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13
Add the toasted panko to the pan with the remaining onions, season with salt and pepper, and stir to combine.
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14
Return the panko-onion mixture to the small bowl and set aside.
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15
Wipe out the frying pan with paper towels, add the remaining 6 tablespoons of butter, and melt over high heat until foaming.
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16
Add the mushrooms and cook, stirring occasionally, until browned and the liquid from the mushrooms is almost all evaporated, about 5 to 6 minutes.
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17
Reduce the heat to medium and add the reserved 1/4 cup of onions to the pan.
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18
Sprinkle with the flour, season with salt and pepper, and stir to coat the vegetables.
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19
Cook, stirring frequently, until the raw taste of the flour is cooked off, about 1 minute.
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20
Gradually whisk in the milk, add the measured salt, and bring to a simmer.
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21
Continue simmering, stirring often, until the mushroom mixture has thickened slightly, about 1 to 2 minutes more.
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22
Transfer the mushroom mixture to the reserved large pot, add the reserved green beans, and stir to combine.
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23
Taste and season with salt and pepper as needed.
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24
Transfer to a 13-by-9-inch baking dish and bake for 10 minutes.
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25
Remove the casserole from the oven, evenly sprinkle the reserved panko-onion mixture across the top, and continue baking until the casseroles bubbling around the edges and heated through, about 10 to 15 minutes more.
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26
Place the pan on a wire rack and let cool 10 minutes before serving.
