Preheat your oven to 475 F degrees. Spray a baking sheet with cooking spray.
Add the flour, breadcrumbs and salt to a bowl and mix. Add the onions and toss them in the flour mixture. Spread the onions over the baking sheet and bake for 30 minutes or until crispy and golden brown.
Reduce the oven heat to 400 F degrees.
Green Beans Casserole
While the onion is in the oven, start by blanching the beans. Add about a gallon of water and a tsp of salt to a large pot and bring it to a boil. Add the green beans to the pot and blanch for 5 minutes. Remove the beans from the boiling water and plunge them into a large bowl of ice water, to stop the cooking. Drain and set aside.
Melt the 2 tbsp of butter in a large skillet over medium heat. Add the mushrooms, season with salt and pepper and cook for about 5 minutes, stirring occasionally.
Add the minced garlic and nutmeg to the skillet and cook for another minute or 2 until the garlic becomes fragrant.
Sprinkle the 2 tbsp of flour over the mushrooms and stir to combine well. Add the chicken broth, stir with a wooden spoon and cook for a minute. Decrease the heat to medium-low and add the heavy cream. Continue cooking for another 6 to 8 minutes until the mixture thickens.
Turn off the heat and add the green beans, chopped bacon and a quarter of the crispy onion. Stir well then top with remaining crispy onions.
Place the skillet in the oven and bake for 15 minutes until bubbly.
Make-ahead instructions: Prepare everything as directed, except turn off the oven after the onions are done. Once the casserole is finished and assembled allow it to cool completely. Cover tightly with foil or in an airtight container and refrigerate until ready to serve. Before serving bake it in the skillet for about 20 minutes or until bubbly at 400 F degrees.
Store leftover green bean casserole in an airtight container in the fridge for 3 to 4 days.
To freeze, store the casserole in an airtight container for up to 6 months.