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1
Preheat oven to 400u00b0F.
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2
Spray an 8-to-9-inch square casserole dish with nonstick cooking spray. Set aside.
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3
Cut green beans into 1 1/2-inch pieces. Slice mushrooms, finely chop onion, and mince garlic and set aside.
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4
Boil large pot of salted water. Once water is boiling, add green beans. Blanch green beans until bright green, about 3 minutes. Drain green beans in a colander and rinse with cold water.
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5
Set large pot over medium-high heat and add butter. Once butter is melted, add onions and mushrooms to pot. Saute until mushrooms are browned and shrunken and onions are soft, 6-7 minutes. Add garlic and thyme and cook until fragrant, 30 seconds.
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6
Sprinkle flour over mushrooms and onions. Stir with spatula to coat. Slowly pour chicken broth into pot, whisking to dissolve flour. Pour in half-and-half and season with salt and black pepper.
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7
Cook sauce, whisking constantly, until it begins to bubble and thicken. Reduce heat to medium-low and allow sauce to simmer, whisking occasionally, until it is reduced by about a third, 5-7 minutes.
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8
Add green beans to pot and stir to coat with sauce. Pour green bean mixture into casserole dish.
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9
Bake casserole on middle rack of oven for 15 minutes.
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10
Sprinkle french-fried onions in even layer over top. Continue baking until casserole is bubbling at edges, 5-10 minutes.
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11
Check to see that casserole is done. Remove from oven or add time as needed.
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12
Allow to rest for 10-15 minutes before serving.
