1. Preheat the oven to 350°F (175°C) and lightly grease a 9×13-inch glass baking dish.

2. In a large pot of salted boiling water, blanch the green beans for 4–5 minutes until bright green and tender-crisp, then transfer to an ice bath and drain.

3. In a skillet over medium heat, melt butter and sauté onion, garlic, and mushrooms until soft, about 5–7 minutes.

4. Stir in flour to form a roux, cook 1 minute, then gradually whisk in milk or half-and-half until smooth. Season with salt and black pepper.

5. Fold the blanched green beans into the mushroom sauce, add cream of mushroom soup if using, and spread evenly in the prepared glass dish.

6. Sprinkle fried onions over the top and add shredded cheese if desired.

7. Bake uncovered for 25–30 minutes until the sauce bubbles and the topping is golden brown.

8. Let rest for 5–10 minutes before serving to allow the casserole to set.