Preheat the oven to 350°F. Lightly grease a 9x13-inch baking dish. Bring a large pot of salted water to a boil. Add the 1 1/2 pounds of green beans and cook for 4 to 5 minutes, until bright green and just tender. Drain, pat dry, and set aside. (Using frozen or canned beans? See Notes.)
In a large skillet, melt the 4 tablespoons of butter over medium heat. Add the 8 ounces of chopped mushrooms and 1/2 cup of chopped sweet onion. Cook for 5 to 7 minutes, until softened. Add the 2 cloves of minced garlic and cook for 1 minute.
Sprinkle the 1/4 cup of flour over the vegetables and stir until evenly coated. Slowly whisk in the 1 1/4 cups of broth until smooth, followed by the 1 cup of heavy cream, over medium heat. Add the 1/2 teaspoon of kosher salt, 1/2 teaspoon of garlic powder, and 1/4 teaspoon of black pepper. Cook for 4 to 6 minutes, stirring often, until the sauce thickens.
Add the green beans to the skillet and gently stir until evenly coated in the creamy mushroom sauce. Transfer to the prepared baking dish and spread into an even layer.
Cover the dish with foil and bake for 30 minutes. Remove the foil and sprinkle the 6 ounces of crispy fried onions evenly over the top. If using the panko topping instead, prepare it according to the topping instructions and sprinkle it evenly over the casserole.
Return to the oven and bake, uncovered, for 5 to 10 minutes for crispy onions or 10 minutes for the panko topping, until the casserole is hot and bubbling and the topping is golden and crisp. If it starts to brown too quickly, remove it early.
Let the casserole rest for 5 to 10 minutes before serving.