Blanch the green beans:Heat large pot of salted water to boil. Boil the green beans for about 5 minutes or until your tender. Drain and put in ice bath to stop the cooking.
Make the mushroom sauce:In oven proof skillet, melt 2 Tablespoons of butter over medium-high heat. Add mushrooms and cook until golden and excess mushroom liquid evaporates, about 5 minutes.Add garlic powder or garlic and remaining butter (2 tablespoons). Cook for about 1 minute.
Stir in the flour, cooking and stirring to remove lumps. Add the stock and Worcestershire sauce whisking until incorporated.Add the milk. Whisk until well combined. Season with salt and pepper, to taste (the saltiness and flavors will concentrate some when it reduces down).
Reduce heat to medium, stirring often, simmer until it starts to thicken, about 5 minutes. Taste for seasoning and salt again if needed. Turn off the heat.
Finish the Casserole: Add the green bean and cheese to the mushroom sauce in that same oven-safe skillet or combine them in a baking dish. Top the casserole with bread crumbs and fried onions.
Bake the casserole for about 15 minutes or until it’s golden brown and bubbly.