8ouncesbaby bella (cremini) mushroomssliced (do not use plain white mushrooms, as they do not have much flavor)
7ounces2% Greek yogurt
Position a rack in the upper third of your oven and preheat to broil. Lightly coat a 2-quart, broiler-safe baking dish with nonstick spray.
In a small bowl, stir together the Panko, Parmesan, parsley, and 2 tablespoons olive oil. Set aside.
Blanch the green beans: Bring a very large pot of water to a boil. Prepare an ice water bath and set out a large, clean kitchen towel. Place the beans in the water and cook just until crisp-tender, about 3 minutes. Plunge into an ice bath to stop the cooking, then drain and transfer to the towel. Lightly pat dry.
Meanwhile, heat 1 tablespoon oil in a large, wide saucepan over medium heat. Add onion and mushrooms. Cook, stirring frequently, until the onion is soft and golden and the mushrooms brown and have given up their liquid, about 15 minutes. Add the remaining 1 tablespoon olive oil. Sprinkle the flour over the top. Cook, stirring, for 1 to 2 minutes more, until all of the flour turns golden and no white bits remain.
Slowly add the milk a few splashes at a time, stirring between each addition to prevent lumps from forming. Increase the heat to medium high. Stir in the salt, pepper, and nutmeg. Cook and stir, allowing the sauce to bubble, running a wooden spoon or spatula along the bottom of the pan. Continue cooking and stirring until the sauce reduces and thickens to resemble a creamy gravy, about 8 to 10 minutes. Remove from the heat and stir in the Greek yogurt.
Transfer half the green beans to the prepared baking dish. Spread half the sauce over the green beans. Add the remaining green beans and top with the remaining sauce. Sprinkle the breadcrumb mixture over the top.
Broil, watching closely, until the casserole is bubbling and beginning to brown on top, 1 to 3 minutes, depending on your broiler. Let stand for 10 minutes prior to serving. Garnish with additional fresh parsley.
Green beans can be blanched 1 day in advance and the sauce prepared 1 day in advance. Store separately in the refrigerator, then gently reheat the sauce on the stove until steaming before combining with the green beans. If the mixture becomes too thick, thin it out with a splash or two of milk. I haven’t tested assembling the entire casserole in advance, but I think that you could layer together the sauce and beans in the baking dish, cover the dish with foil, then heat it (covered) in a 350 degree F oven until hot. From there, sprinkle with breadcrumbs and broil closer to serving.