For the green beans: Melt 8 tablespoons of the butter in a large saucepan over low heat. Add the onions and stir to coat thoroughly with the butter. Cook, covered, for 30 minutes; you don¿t want the onions to take on any color so you can check them occasionally, but don't stir. After 30 minutes, stir in 2 pinches salt and cook, covered, stirring every 8 to 10 minutes so the onions don't stick to the bottom of the pot, until they are very soft, about 30 minutes more. There should still be no color on the onions or the bottom of the pot.
While the onions cook, bring a large pot of salted water to a boil over high heat. Working in batches so you don't crowd the pot, add the green beans and cook 30 seconds. Using a skimmer, immediately transfer the green beans to a bowl of ice water to cool. Drain well; transfer to a baking sheet lined with paper towels to absorb excess water. Set aside.
Put the remaining 8 tablespoons butter in a large skillet over medium-high heat; when it turns light brown and smells nutty, add the cremini mushrooms, season with a pinch of salt, and cook, stirring, until they turn deep golden brown, about 10 minutes. Set aside.
Preheat the oven to 350 degrees F. After the onions have cooked 1 hour, stir in the flour. The mixture (or roux) will start to form a lumpy mass; don¿t worry, once you add the onion soup, the mixture will loosen up. Stir in the soup until completely incorporated. Cook, stirring gently, to cook out the raw flour taste, about 3 minutes. Add the Cheddar and whisk until it melts and the mixture has thickened.
Stir the cremini mushrooms into the cheese sauce; season with salt and pepper. Stir in the green beans. Pour the green bean and mushroom mixture into a 9-by-13 inch casserole dish. (The casserole can be made up to his point 1 day ahead, and refrigerated, covered. Let come to room temperature before continuing.) Cover with foil and bake for 35 minutes.
For the topping: Meanwhile, melt the butter in a large skillet over medium heat. Add the shiitakes and cook, stirring, until crisp and golden brown, about 3 minutes. Transfer the shiitakes, along with any butter remaining in the skillet, to a food processor and pulse until coarsely ground. Add the panko and Parmesan and pulse to combine. Transfer the topping to a bowl and stir in the chives and parsley.
After the casserole has cooked for 35 minutes, sprinkle the crumb mixture evenly over the casserole and bake, uncovered, until the topping is golden brown, about 20 minutes. Serve in the casserole dish, family-style.
Copyright 2014 Cooking Channel, LLC. All rights reserved.