Step 1

In a large skillet, heat 4 Tablespoons of butter over medium-high heat. Add onions and cook until tender and caramelized, about 7-8 minutes, stirring often. Add mushrooms and cook for 4 minutes longer. Stir in garlic and cook for 1 minute. Transfer the mixture to a bowl.

Step 2

After the onion-mushroom mixture has been removed, use the same skillet. Heat skillet to a medium heat and add remaining 4 Tablespoons of butter. Add flour or cornstarch and begin to quickly whisk the mixture. Let cook, whisking often, for about 2 minutes.  Stir in half-n-half, salt, and pepper. Cook for 4-5 minutes until mixture thickens. Taste and check for seasonings.

Step 3

Preheat oven to 350 degrees. Bring a large pot of water to a boil. Add green beans and cook for 6-7 minutes.  Drain and transfer to ice bath to cool (optional).

Step 4

Add onion-mushroom mixture to cream sauce and stir. Add green beans and stir to coat. Sprinkle with parmesan cheese, if using. Pour into a 9 x 9 baking dish. Bake for 25 minutes or until bubbly.  Remove from the oven and cover with fried onions. Bake for 5 minutes longer. Serve immediately, if possible.