Preheat oven to 400 degrees F.
Bring a large pot of slated water to a boil over medium heat. Add the beans and cook for 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 to stop the cooking. Drain the beans and set aside.
Melt 2 TB of the butter in a large skillet over medium-high heat. Add the onions and saute until translucent. Stir in the mushrooms, season with salt and pepper, to taste, and saute about 3 to 5 minutes. Add the flour and stir for another minute, to cook out the raw flour taste, and then whisk in th sour cream. Simmer for just a couple of minutes to bring the flavors togehter and taste for seasoning.
Cut the green beans into 2-inch pieces. Melt the remaining 2 TB of butter in a small pan. Crush the cereal flakes in a resealable plastic bag and add them to the pan with the butter. Toss to coat.
Spread the green beans over the bottom of a 9-inch casserole dish.
Pour all of the mushroom sauce mixture over the beans and top with the grated cheese.
Sprinkle with the butter cereal flakes and bake until light golden brown and bubbly, approsimately 20 minutes.
Remove from the oven and serve.