Preheat the oven to 350°F (175°C).
In a medium mixing bowl, combine cream of mushroom soup, whole milk, soy sauce, salt, and black pepper. Whisk until smooth.
Add the green beans and gently fold everything together until the green beans are evenly coated.
Transfer the mixture to a 1 ½-quart baking dish and spread it out evenly with a spatula.
Bake uncovered for 25 minutes, or until the mixture is hot and bubbling.
Carefully remove the dish from the oven and sprinkle the French fried onions evenly over the top.
Return the casserole to the oven and bake for an additional 5 to 10 minutes, or until the onions on top are golden brown and crispy.
Let the casserole rest for 5 minutes before serving.