1. Blanch the Green Beans: Bring a large pot of salted water to a boil. Add the fresh green beans and blanch for 2-3 minutes until they are bright green and slightly tender. Drain and immediately plunge them into a bowl of ice water to stop the cooking process. Drain again and set aside.
   Preheat the Oven:
2. Preheat your oven to 400°F (204°C).
3. Prepare the Mushroom Sauce: In a large skillet, melt the butter over medium heat. Add the chopped onions and cook for about 3-4 minutes until they start to become translucent. Add the minced garlic and sliced mushrooms to the skillet. Cook for another 5-7 minutes until the mushrooms have released their moisture and are slightly browned. Season the mushroom mixture with salt and black pepper to taste.
4. Make the Creamy Sauce: Sprinkle the 3 tablespoons of flour over the mushroom mixture. Stir well to combine and cook for 2-3 minutes. Slowly pour in the half and half (or milk and cream mixture) while whisking constantly to prevent lumps. Cooking and stirring until the sauce thickens, about 5-7 minutes. Stir in the chicken or vegetable broth and simmer, stir a few times, for another 5 minutes until the sauce is smooth and creamy. Adjust salt and pepper to taste.
5. Combine the Sauce and Green Beans: Add the blanched green beans to the skillet with the mushroom sauce. Gently stir to coat the green beans evenly with the sauce.
6. Assemble and Bake: Transfer the green bean and mushroom mixture to a greased 9×13-inch baking dish. Sprinkle the panko and fried onion topping evenly over the green beans.
7. Bake: Place the casserole in the oven for 20 minutes, or until the topping is golden brown and the casserole is bubbling.
8. Serve: Remove from the oven and let it cool slightly before serving. Enjoy your homemade green bean casserole!

This green bean casserole will totally be a hit at your next gathering or holiday meal!