Bring a large pot of water (about 2 quarts) to a boil with 2 T. kosher salt (large flake). Add the beans and boil for 5 minutes. Drain in a colander and place beans in a large bowl of ice water. Drain and set aside.
Melt the butter in a cast iron skillet over medium-high heat. Add the mushrooms, 1 teaspoon salt and pepper and cook the mushrooms for about 5 minutes. Add the garlic, nutmeg, cajun seasoning and continue to cook for another couple minutes.
Add the flour and stir to combine. Cook for about a minute. Add the broth and simmer for a minute. Decrease the heat to medium-low and add the half-and-half. Cook until the mixture thickens and coats the back of a spoon.
Remove from the heat and stir in 1/2 cup of the onions and the green beans. Top with the remaining cup of onions. Place in a 400 degree oven and bake for about 20 minutes. Say Grace and enjoy immediately!