Preheat the oven to 475 degrees F. Put the onions, flour, panko and salt in a large mixing bowl and toss to combine.
Bake the Onions
Coat a sheet pan with nonstick cooking spray and evenly spread the onions on the pan. Then, place the pan on the middle rack of the oven and bake until golden brown, approximately 30 minutes. Toss the onions 2 to 3 times during cooking. Once done, remove from the oven and set aside until ready to use. Turn the oven down to 400 degrees F.
Prepare the Beans
While the onions are cooking, prepare the beans. Bring a gallon of water and 2 tablespoons of salt to a boil in an 8-quart saucepan. Add the beans and blanch for 5 minutes.
Drain the Beans
Drain in a colander and immediately plunge the beans into a large bowl of ice water to stop the cooking. Then, drain and set aside.
Add the Mushrooms
Melt the butter in a 12-inch cast-iron skillet set over medium-high heat. Add the mushrooms, 1 teaspoon salt and pepper and cook, stirring occasionally, until the mushrooms begin to give up some of their liquid, approximately 4 to 5 minutes. After, add the garlic and nutmeg and continue to cook for another 1 to 2 minutes.
Sprinkle in Flour
Cook for 1 minute. Add the broth and simmer for 1 minute. Sprinkle the flour over the mixture and stir to combine.
Cook the Mushrooms
Decrease the heat to medium-low and add the half-and-half. Cook until the mixture thickens, stirring occasionally, approximately 6 to 8 minutes.
Stir in Onions and Green Beans
Remove from the heat and stir in 1/4 of the onions and all of the green beans.
Bake and Serve
Top with the remaining onions. Place in the oven and bake until bubbly, for approximately 15 minutes. Remove and serve immediately.
