Preheat the oven to 350°F (175°C).
In a large skillet, melt the butter over medium heat.
Add the sliced mushrooms and sauté until soft.
Stir in the flour and cook for one minute.
Gradually whisk in the vegetable broth and heavy cream.
Add soy sauce, salt, and pepper to taste.
Combine the green beans in a mixing bowl with the sauce.
Transfer the mixture to a greased baking dish.
Bake for 25 minutes, then top with crispy fried onions.
Return to the oven and bake for an additional 10-15 minutes, until golden brown.