Slice onion very thin. Place in a baking dish and cover with buttermilk and soak for at least an hour. Combine dry ingredients and set aside. Heat oil to 375 degrees. Grab a handful of onions, throw into the flour mixture, tap to shake off excess, and PLUNGE into hot oil. Fry for a few minutes and remove as soon as golden brown. Lay on a paper towel-lined baking sheet.  Repeat until onions are gone.

Preheat the oven to 475 degrees F.  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. 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.  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.