Preheat the oven to 475 degrees F.
Combine the onions
, flour, panko and salt in a large mixing bowl and toss to combine. Coat a sheet pan
with nonstick cooking spray
and evenly spread the onions on the pan. 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.
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 in a colander
and immediately plunge the beans into a large bowl of ice water to stop the cooking. Drain
and set aside.
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. Add the garlic and nutmeg
and continue to cook for another 1 to 2 minutes. Sprinkle the flour over the mixture and stir to combine. Cook for 1 minute. Add the broth and simmer
for 1 minute. Decrease the heat to medium-low and add the half-and-half. Cook until the mixture thickens, stirring occasionally, approximately 6 to 8 minutes.
Remove from the heat and stir in 1/4 of the onions and all of the green beans
. Top with the remaining onions. Place into the oven and bake until bubbly, approximately 15 minutes. Remove and serve immediately.