Bring a large pot of salted 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 stop the cooking. Drain the beans and set aside.
Melt 2 tablespoons 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 the sour cream. Simmer for just a couple of minutes to bring the flavors together and taste for seasoning.
Cut the green beans into 2-inch pieces. 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. 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 buttered cereal flakes and bake in the preheated 400 degree F oven, until light golden brown and bubbly, approximately 20 minutes. Remove from the oven and serve.