Preheat the oven to 450 degrees F.
Saute onions over medium-low heat with olive oil and 1 tsp of kosher salt. You will need to stir regularly but not to often because it will prevent caramelization. Once the onions are a rich brown color remove from heat and set aside.
Spread panko on a sheet pan and bake for 10-15 minutes or until lightly golden brown.
While Bring a gallon of water and 3 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.
Heat a 12-inch cast iron skillet or uncoated stainless steel pan set over medium-high heat and add bacon pieces. Stir bacon occasionally until evenly browned. Remove bacon from pan and set aside but leave bacon fat in pan.
Add half the mushrooms, 1 teaspoon salt and pepper and cook, stirring occasionally. When the mushrooms start to brown add the second half of the mushrooms. When the mushrooms begin to give up some of their liquid and are starting have a consistent brown, 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, 1/2 the bacon and all of the green beans. Top with a blend of the remaining onions, bacon and panko. Place into the oven and bake until bubbly, approximately 15 minutes. Remove and serve immediately.