Preheat oven to 350°F (175°C).
Bring a large pot of salted water to a boil. Add green beans and cook for 4-5 minutes until crisp-tender. Drain and immediately transfer to a bowl of ice water to stop cooking. Drain again and pat dry.
In a large bowl, whisk together the condensed cream of mushroom soup, whole milk, soy sauce, black pepper, garlic powder, and onion powder until smooth.
Add the blanched green beans and 2/3 cup of French fried onions to the mushroom sauce mixture. Stir gently to combine.
Transfer the mixture to a 9×13 inch casserole dish and spread evenly.
Cover the casserole dish tightly with aluminum foil or plastic wrap.
To freeze: Place the covered casserole in the freezer for up to 3 months.
To bake from frozen: Preheat oven to 350°F (175°C). Remove plastic wrap if used and cover with foil. Bake for 40 minutes.
Remove foil and sprinkle the remaining 2/3 cup French fried onions evenly over the top of the casserole.
Bake uncovered for an additional 10-15 minutes, or until the onions are golden brown and the casserole is bubbly.
Let the casserole rest for 5 minutes before serving.