Preheat the Oven: Preheat your oven to 350°F (175°C).
Prepare the Green Beans: If you're using fresh green beans, trim the ends and blanch them in boiling water for about 3-4 minutes until just tender. Drain and set aside. If using canned green beans, simply drain and rinse them.
Make the Mushroom Sauce: In a large mixing bowl, combine the cream of mushroom soup, milk, garlic powder, salt, and pepper. Stir well to combine until smooth.
Assemble the Casserole: Add the prepared green beans to the mixing bowl with the mushroom sauce. Stir until the green beans are fully coated with the creamy sauce.
Transfer to Baking Dish: Pour the green bean mixture into a 9x13-inch baking dish. Spread it out evenly.
Add the Topping: Sprinkle the fried onions evenly over the top of the casserole.
Bake: Place the casserole in the preheated oven and bake for 25-30 minutes, or until the casserole is bubbly and the onions are golden brown and crispy.
Serve: Allow the casserole to cool for a few minutes before serving. Enjoy!