1. Prepare the Green Beans: Begin by bringing a large pot of salted water to a boil. Add the trimmed fresh green beans and cook for 5-7 minutes, or until they are tender-crisp. You want them to still have a slight bite. Immediately drain the green beans and plunge them into an ice bath to stop the cooking process and preserve their vibrant green color. Once cooled, drain thoroughly and set aside.

2. Saute the Aromatics: In a large oven-safe skillet or Dutch oven, melt the butter over medium heat. Add the finely chopped yellow onion and sliced mushrooms. Cook, stirring occasionally, for 8-10 minutes, until the onions are softened and translucent, and the mushrooms have released their liquid and browned nicely. This step builds a foundational layer of flavor for your sauce.

3. Make the Roux: Sprinkle the all-purpose flour over the cooked onions and mushrooms. Stir constantly for 1-2 minutes, allowing the flour to cook and form a roux. This will thicken your sauce later. The mixture should be golden and fragrant, like a paste.

4. Whisk in Liquids and Seasonings: Gradually whisk in the whole milk and chicken or vegetable broth, a little at a time, ensuring no lumps form. Continue whisking until the sauce is smooth and begins to thicken, about 3-5 minutes. Stir in the salt, black pepper, and garlic powder. Taste and adjust seasonings as needed. The sauce should be creamy and well-seasoned.

5. Combine and Add Cheese: Remove the skillet from the heat. Gently stir in the blanched green beans and 3/4 cup of the shredded sharp cheddar cheese into the creamy sauce. Make sure the green beans are evenly coated.

6. Bake the Casserole: Smooth the green bean mixture into an even layer in the skillet or transfer it to a 9×13 inch baking dish. Sprinkle the remaining 1/4 cup of shredded cheddar cheese over the top. Bake in a preheated oven at 375 F (190 C) for 15 minutes, or until the casserole is bubbly around the edges and the cheese is melted.

7. Add Crispy Onions and Finish Baking: Remove the casserole from the oven and evenly sprinkle the crispy fried onions over the top. Return to the oven and bake for an additional 5-10 minutes, or until the fried onions are golden brown and crispy. Keep a close eye on them to prevent burning.

8. Serve Warm: Let the casserole rest for a few minutes before serving to allow the sauce to set slightly. Serve hot and enjoy!