1. Bring a large pot of salted water to a boil and blanch green beans for 3–4 minutes until bright and slightly tender, then shock in ice water and drain.

2. Preheat the oven to 375°F (190°C) and grease a 13×9-inch baking dish.

3. In a skillet over medium heat, melt butter and sauté sliced mushrooms for 5–6 minutes until browned, then stir in flour and cook for 1 minute.

4. Gradually whisk in milk, cream, and broth, cooking 3–5 minutes until sauce thickens; season with onion powder, salt, and pepper.

5. In a mixing bowl, toss drained green beans with the sauce and half of the fried onions, then spread mixture in the baking dish and top with remaining onions and optional Parmesan or breadcrumbs.

6. Bake for 25–30 minutes until bubbly and golden, then let rest 10 minutes before serving; broil 1–2 minutes if extra crisp topping is desired.