1. Prepare the Green Beans: Bring a large pot of salted water to a boil. Add the trimmed green beans and cook for 5-7 minutes, or until they are tender-crisp. You want them bright green and slightly softened but still with a little bite. Drain them immediately and plunge them into an ice bath to stop the cooking process. This helps retain their vibrant color and crisp texture. Once cooled, drain thoroughly and set aside.

2. Saute Aromatics and Mushrooms: In a large oven-safe skillet or Dutch oven, melt the butter over medium heat. Add the finely chopped yellow onion and cook for 3-5 minutes until it softens and becomes translucent. Add the sliced mushrooms and continue to cook for another 5-7 minutes, stirring occasionally, until the mushrooms release their liquid and start to brown.

3. Create the Roux: Sprinkle the all-purpose flour over the cooked onions and mushrooms. Stir constantly for 1-2 minutes to cook out the raw flour taste, creating a roux. It should form a thick paste.

4. Whisk in Liquids and Seasonings: Gradually whisk in the chicken broth, a little at a time, ensuring no lumps form. Once all the broth is incorporated, slowly whisk in the heavy cream. Bring the mixture to a gentle simmer, stirring constantly, until the sauce thickens to a creamy consistency. Season with garlic powder, black pepper, and salt. Taste and adjust seasoning as needed.

5. Combine and Bake: Gently stir the blanched green beans into the creamy mushroom sauce. Fold in about two-thirds of the crispy fried onions. Pour the mixture into a 9×13 inch baking dish if not using an oven-safe skillet.

6. Final Bake: Bake in a preheated oven at 375 F (190 C) for 20-25 minutes, or until the casserole is bubbly around the edges. Remove from the oven, sprinkle the remaining crispy fried onions over the top, and return to the oven for another 5 minutes, or until the onions are golden brown and crisp. Serve hot.