Preheat the oven to 350?F. Lightly grease a 9 x 13-inch casserole with 2 teaspoons butter and set aside.
Heat at least 4 inches oil to 360?F in a large saucepan or deep fryer.
Separate the onion slices into individual rings. Combine the onion rings with the Crystal Hot Sauce in a mixing bowl and toss thoroughly. Place 2 cups of the flour in a large bowl and, working in batches, dredge the onion rings in the flour to coat. Transfer the coated onion rings to a colander or strainer and shake over the bowl containing the flour to release any loose flour. Fry the onion rings in batches until just lightly golden, about 1 minute per batch. As batches are completed, transfer the fried onion rings to a paper towel-lined baking dish to drain. Season the onion rings with the Essence and set aside. Note: It is important to allow the temperature to return to 360?F between batches.
Melt the remaining 6 tablespoons butter in a large saucepan over high heat and cook the chopped onions and celery until soft, about 6 minutes. Add the garlic and cook, stirring, for 1 minute. Add the mushrooms, Bayou Blast, and salt and cook, stirring frequently, until the mushrooms are soft and golden brown and have released their liquid, 4 to 6 minutes. Sprinkle with the remaining 1/4 cup flour and stir to combine. Cook for 2 minutes, stirring constantly. Add the Chicken Stock and cream and continue to cook, stirring frequently, until the sauce is thick and creamy and any floury taste is gone, about 15 minutes. Remove from the heat.
In a large bowl, combine the green beans, mushroom sauce, and cubed Fontina cheese. Transfer to the prepared casserole dish and top with the fried onion rings. Bake for 20 to 25 minutes, or until hot and bubbly and the onion rings are golden brown. Serve immediately.
Makes 8 to 10 servings