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1
Preheat the oven to 400 degrees F.
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2
Bring a large pot of salted water to a boil over medium heat then add the beans and cook until just tender (but still firm), about 5 to 7 minutes. Remove the beans from the water to a large bowl of ice water to preserve color and stop the cooking then drain the beans and set aside.
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3
Melt 2 tablespoons of the butter in a large skillet over medium-high heat; Add the onions and saute until translucent.
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4
Stir in the mushrooms, season with salt and pepper to taste, saute about 3 to 5 minutes.
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5
Add the flour and stir for another minute, to cook out the raw flour taste, and then whisk in the sour cream.
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6
Simmer for just a couple of minutes to bring the flavors together and taste for seasoning.
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7
Cut the green beans into 2-inch pieces.
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8
Melt the remaining 2 tablespoons of butter in a small pan. Crush the cereal flakes in a resealable plastic bag and add them to the pan with the butter, tossing to coat.
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9
Spread the green beans over the bottom of a 9-inch casserole dish and pour all of the mushroom sauce mixture over the beans topping with the grated cheese.
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10
Sprinkle with the buttered cereal flakes and bake in the preheated 400 degree F oven, until light golden brown and bubbly, approximately 20 minutes.
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11
Remove from the oven and serve.
