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1
Preheat oven to 400F.
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2
Coat a 2 1/2-quart baking dish evenly with cooking spray or grease with butter.
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3
Heat 1 tablespoon oil in a large saucepan over medium heat.
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4
Stir in diced onion and cook, stirring often, until softened and slightly translucent, 3 to 5 minutes.
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5
Add mushrooms, onion powder, 1 teaspoon salt, thyme and pepper.
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6
Cook, stirring often, until the mushroom juices are almost evaporated, about 4 minutes.
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7
Sprinkle 13 cup flour over the vegetables, stir to coat.
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8
Stir in milk and sherry and bring to a simmer, stirring often.
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9
Add green beans and bring to a simmer again.
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10
Cook, stirring, until heated through, 1 to 2 minutes.
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11
Stir in sour cream and buttermilk powder.
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12
Transfer to the prepared baking dish.
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13
Mix together the remaining 13 cup flour, paprika, garlic powder and the remaining 1/4 teaspoon salt in a shallow dish.
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14
Add sliced onion, toss to coat.
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15
Heat the remaining 2 1/2 tablespoons oil in a large nonstick skillet over medium-high heat.
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16
Stir in the onion along with any remaining flour mixture and cook, turning once or twice, until golden and crispy, about 5 minutes.
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17
Spread the onion topping evenly over the casserole.
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18
Bake the casserole until bubbling, about 15 minutes.
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19
Allow cool for a few minutes.
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20
Serve warm.
