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1
Preheat the oven to 350 degrees F.
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2
(If you haven't steamed your green beans, begin with those they'll be done, about 20 minutes, by the time you need to mix them into the casserole.
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3
).
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4
Begin the sauce: in a saucepan melt the butter over low heat, then whisk in the flour, stirring continually, and cook for 2 minutes (the mixture will be quite thick).
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5
Gradually add the milk while whisking, and bring to a simmer; cook, whisking continually, for 3 or so minutes and it thickens.
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6
Add the concentrated chicken base, celery salt, nutmeg, and white pepper; whisk well to make sure chicken base is dissolved, remove from heat and set aside.
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7
Begin the filling: in a large nonstick skillet, melt together the olive oil and butter over medium heat; add the onion and garlic, and saute until onion begins to soften, about 5 to 7 minutes.
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8
Add the chopped mild green chiles, stir well to combine, and let the mixture brown for about 3 minutes without stirring.
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9
Add the sliced mushroom, cover, and saute, stirring occasionally, until mushrooms are tender, about 5 minutes; remove about half of the mushroom mixture from the pan and set it aside.
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10
In a 1 1/2 quart lightly-sprayed casserole dish, combine the half of the mushroom mixture from the pan, the cooked green beans, and the white sauce; taste test and adjust salt and pepper, if necessary.
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11
Combine the remaining mushroom mixture with the bread crumbs; combine the melted butter and the sage, then stir it into the crumb mixture.
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12
Spread the crumb mixture over the beans.
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13
Bake uncovered for 30 minutes.
