Preheat oven to 475ºF.
In a large mixing bowl, combine the all-purpose flour, Panko bread crumbs and kosher salt for the topping. Add in the sliced onion and toss to coat.
Rub a large sheet plan with a tablespoon of unsalted butter and spread the onions evenly.
Put the sheet pan in the oven on the middle rack and bake until golden brown, about 30 minutes. Toss the onions 2-3 times during cooking. When done, remove the onions from the oven and set aside. Turn down the oven to 400ºF.
While the onions are in the oven, prepare the beans. In a large saucepan, bring water and 2 tablespoons of kosher salt to a boil, then add the green beans and blanch them for 5 minutes.
Drain the beans and immediately plunge them into a large bowl of ice water to stop the cooking. Drain the beans again and set them aside until ready to use.
In a large skillet, melt 2 tablespoons of unsalted butter over medium-high heat. Add the sliced mushrooms, 1 teaspoon of kosher salt and the black pepper. Cook, stirring occasionally, until the mushrooms begin to give up some of their liquid, about 4-5 minutes. Add the minced garlic and nutmeg and continue to cook for another 1-2 minutes. Sprinkle the mushrooms with the flour and stir to combine. Cook for another minute. Pour in the chicken stock and simmer for a minute. Decrease the heat to medium-low and add the half-and-half. Continue to cook until the mixture thickens, stirring occasionally, about 6-8 minutes.
Remove the skillet from the heat and stir in ¼ of the baked onions and all of the green beans. Transfer the mixture into a 9"x13” baking dish and top with the remaining onions.
Bake in the oven at 400ºF until bubbly, about 15 minutes. Remove and serve immediately.