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

Blanch the green beans: Bring a large pot of salted water to a boil. Add the green beans and cook for 3-4 minutes until bright green and slightly tender. Drain and immediately plunge into ice water to stop the cooking process. Drain again and set aside.

Prepare the creamy mushroom sauce: In a large skillet, melt the butter over medium heat. Add the chopped onion and cook for 3-4 minutes until softened. Stir in the garlic and cook for another 30 seconds until fragrant.

Cook the mushrooms: Add the sliced mushrooms to the skillet and cook until they release their moisture and become golden brown, about 5-6 minutes.

Make the sauce: Sprinkle the flour over the mushroom mixture and stir well to coat. Cook for 1-2 minutes to remove the raw flour taste. Gradually whisk in the broth and heavy cream until smooth and thickened. Season with salt, pepper, and paprika. Simmer for 2-3 minutes until the sauce has a creamy consistency.

Combine green beans and sauce: Add the blanched green beans to the mushroom sauce and stir until evenly coated. Pour the mixture into the prepared baking dish.

Add toppings: Sprinkle the French fried onions evenly over the casserole. For an extra layer of flavor, you can also sprinkle Parmesan cheese on top.

Bake: Place the casserole in the preheated oven and bake for 25-30 minutes, or until the top is golden brown and crispy.

Serve: Let the casserole rest for 5 minutes before serving to allow the flavors to settle and make it easier to scoop.