Step 1: Prepare the Green Beans

• Blanch the green beans: Bring a large pot of salted water to a boil. Add the green beans and cook for 4-5 minutes until they are bright green and slightly tender. Drain and immediately transfer the beans to a bowl of ice water to stop the cooking process. Drain again and set aside.

Step 2: Make the Mushroom Sauce

• Sauté the onions and garlic: In a large skillet, melt the butter over medium heat. Add the sliced onion and cook for about 5 minutes, until soft and translucent. Stir in the minced garlic and cook for an additional minute.
• Cook the mushrooms: Add the sliced mushrooms to the skillet and cook for 6-8 minutes until browned and tender.
• Create the roux: Sprinkle the flour over the mushroom mixture and stir to coat. Cook for 1-2 minutes to eliminate the raw flour taste.
• Add broth and cream: Slowly pour in the chicken or vegetable broth, whisking continuously until smooth. Add the heavy cream and continue to whisk until the sauce thickens, about 3-4 minutes.
• Season and add cheese: Stir in the Parmesan cheese, salt, and pepper. Remove from heat.

Step 3: Assemble the Casserole

• Mix the beans and sauce: In a large bowl, combine the blanched green beans with the mushroom sauce. Stir until the beans are evenly coated.
• Transfer to a baking dish: Grease a 9×13-inch baking dish and pour the green bean mixture into the dish, spreading it out evenly.

Step 4: Add the Topping

• Top with crispy onions: Sprinkle the crispy fried onions evenly over the top of the green bean casserole.

Step 5: Bake the Casserole

• Bake: Preheat the oven to 375°F (190°C). Bake the casserole uncovered for 20-25 minutes, or until the sauce is bubbly and the onions are golden brown and crispy.

Step 6: Serve and Enjoy

• Garnish and serve: Let the casserole cool for a few minutes before serving. Serve hot and enjoy this delicious side dish!