Grandma's Green Bean Casserole

Onions for Topping
2 medium onions, thinly sliced
1/4 cup flour
2 Tbsp bread crumbs  (I like to use Panko - she didn't have that luxury)
1 tsp salt

Beans
2 Tbsp plus 1 tsp kosher salt (divided)
1 lb fresh green beans (rinsed, trimmed, and halved)
2 Tbsp butter
12 oz mushrooms (cleaned and sliced)
1/2 tsp freshly ground pepper
2 cloves garlic (minced)
1/4 tsp freshly ground nutmeg
2 Tbsp flour
1 cup chicken broth
1 cup half-and-half

Preheat oven to 475 degrees.

Combine the onions, flour, bread crumbs 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 - 3 times during cooking.  Once done, remove from the oven and set aside until they are ready to use.  Turn the oven down to 400 degrees.

While the onions are cooking, prepare the beans.  Bring a gallon of water and 2 Tbsp of salt to a boil in and 8-quart saucepan.   Add the beans and blanch for 4 minutes.  Drain in a colander and immediately run under cold water to stop the cooking.  Drain and set aside.

Melt the butter in a 12" cast iron skillet set over medium-high heat.  Add the mushrooms, 1 tsp salt and pepper and cook, stirring occasionally, until the mushrooms begin to give up some of their liquid, approximately 4 - 5 minutes.  Add the garlic and nutmeg and continue to cook for another 1 - 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, approximately6 - 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.