I do the whole dish in my 12-inch cast iron skillet and then just throw it in the oven. If you are not using a cast iron pan, you will need a large skillet and a 9 by 9 dish.
Blanch your green beans in boiling salted water for 5 minutes. Add them to ice water to stop cooking and drain.
Heat oven to 350°
In a large skillet, heat 4 Tablespoons of butter over medium-high heat. Add onions and cook until tender, about 5 minutes, stirring often. Add mushrooms and cook for 4 minutes longer. Stir in garlic and cook for 1 minute.
Add the remaining 4 Tablespoons of butter. Add flour and begin to quickly whisk the mixture. Let cook, whisking often, for about 2 minutes. Stir in half-n-half, or cream, stock, worcestershire sauce, hot sauce, nutmeg, salt, and pepper. Cook for 4-5 minutes until mixture thickens. Taste and check for seasonings.
Add green beans and stir to coat. Pour into a 9 x 9 baking dish or leave in the cast iron if using. Cover with the fried onions and bake for 30 minutes or until bubbly.