Preheat the oven to 350°F (175°C).
Bring a large pot of salted water to a boil. Add the green beans and cook for 5 minutes until tender-crisp. Drain and rinse with cold water to stop cooking. Drain again thoroughly.
In a large bowl, combine the condensed cream of mushroom soup, whole milk, soy sauce, black pepper, and garlic powder. Stir until smooth.
Add the cooked green beans and 2/3 cup of French fried onions to the soup mixture. Stir gently to combine.
Transfer the mixture to a 9×13-inch baking dish and spread evenly.
Sprinkle the remaining 2/3 cup of French fried onions evenly over the top.
To freeze: Cover the baking dish tightly with aluminum foil and label with the date.
When ready to bake, preheat oven to 350°F (175°C). Bake the casserole covered for 25 minutes, then remove the foil and bake for an additional 5 minutes until the onions are golden brown and crispy.
Let the casserole sit for 5 minutes before serving.