Preheat the oven to 350°F (175°C).
Prepare the green beans by trimming and blanching them.
Sauté the sliced mushrooms in a pan.
In a mixing bowl, combine the sautéed mushrooms, cream of mushroom soup, and milk.
Add the soy sauce, garlic powder, salt, and pepper to the mixture.
Combine the blanched green beans with the sauce mixture.
Transfer the mixture to a baking dish and spread it out evenly.
Bake the casserole for 25-30 minutes until bubbling.
Top with crispy fried onions and bake for an additional 5-10 minutes.
Let cool slightly before serving.