How to Make It
Preheat the oven to 350°. In a pot of boiling salted water, blanch the haricots verts until crisp-tender, about 2 minutes. Drain and cool under running water. Pat dry.
In a very large, deep skillet, melt 4 tablespoons of the butter. Add the mushrooms, season with salt and pepper and cook over moderately high heat, stirring frequently, until lightly browned, 6 to 7 minutes. Add the onion and garlic and cook over low heat, stirring occasionally, until softened, about 5 minutes. Add the corn and cook, stirring occasionally, until the mixture is dry and the corn is just beginning to brown, about 5 minutes. Add the haricots verts and keep warm.
In a large saucepan, melt the remaining 6 tablespoons of butter. Add the flour and whisk over moderate heat until fragrant and lightly browned, about 4 minutes. Whisk in the milk until smooth and bring to a boil. Reduce the heat to moderately low and simmer, whisking constantly, until very thick, about 5 minutes. Remove from the heat. Stir in the Gruyère and season with salt, pepper and nutmeg. Pour the sauce over the vegetables and stir well. Pour the mixture into a 4-quart baking dish.
In a small bowl, combine the panko with the Parmigiano and sprinkle over the vegetables. Set the baking dish on a rimmed baking sheet and bake in the center of the oven for 30 minutes, until bubbling. Turn on the broiler and broil the casserole for 3 to 4 minutes, until the top is golden and crisp. Let the casserole rest for 10 minutes before serving.