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1
Preheat the oven to 450 degrees F.
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2
Bring a large pot of salted water to a boil.
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3
Add the green beans and cook until they just turn bright green, 2 to 3 minutes.
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4
(You don't want to cook the green beans through as they will cook in the oven also.)
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5
Strain and plunge the green beans immediately in a large bowl of ice water to stop the cooking process.
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6
When cool, drain and set out to dry on paper towels.
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7
Set a 12-inch cast-iron skillet over medium-high heat.
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8
Add the butter and vegetable oil.
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9
Once the butter is melted and the pan is hot, add the mushrooms.
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10
Cook undisturbed so the mushrooms sear nicely, about 1 minute.
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11
Add the thyme and garlic and continue cooking the mushrooms to achieve some nice color.
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12
Season with the salt, pepper, cayenne and nutmeg.
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13
Cook 2 to 3 more minutes longer, dusting the mushrooms with some of the flour as they release moisture.
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14
Stir with a whisk to incorporate the flour (just like making a roux) and gradually add in the chicken stock.
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15
Bring to a simmer, then reduce the heat and add the sour cream and cream.
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16
Stir gently and cook over low heat until gravy thickens, 5 to 6 minutes.
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17
Add the blanched green beans to the pan and fold together so everything is mixed well.
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18
Spread out evenly in the pan.
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19
Sprinkle the green beans with the grated Parmesan and transfer to a roasting tray (to catch any spill-over).
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20
Bake until the casserole is bubbly and the top is melted and golden, 15 minutes.
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21
Pour the vegetable oil into a deep skillet and heat to 350 degrees F. Pour the flour into a large mixing bowl and sprinkle generously with salt and pepper.
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22
Toss the shallot rounds in the seasoned flour, then place in a strainer and shake off any excess flour.
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23
Fry the shallots in the hot oil in small batches until golden brown, 3 to 5 minutes.
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24
Drain on paper towels and season with salt while still hot.
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25
Top the green bean casserole with the fried shallots and serve.
