Preheat the oven to 350°F.
Grease a 2.5 Qt. baking dish with margarine and set aside.
If using fresh green bean, clean and set aside, if using canned green beans, drain and set aside, but if using frozen, no prep is needed.
In a medium sized sauce pan on low heat, melt the margarine with salt and pepper.
Sprinkle in flour while stirring constantly with a wooden spoon.
In 3 to 5 minutes you will have a light roux.
Next pour and blend the milk, soy sauce, and vegetable broth in the roux with a wire whisk.
Turn up the heat to medium and stir constantly for up to 2 minutes or until the sauce thickens.
Once the sauce is thick, set aside.
Clean and cut up the mushrooms, celery, garlic, and onion.
Add the vegetables and oil to a large skillet, saute for about 5 to 7 minutes.
Pour the sauteed vegetables into the white sauce and stir to combine.
Place the green beans into the baking dish, pour the white sauce over the beans and stir.
Next if adding a topping, sprinkle over the top of the beans.
Cover the dish and place into the oven for 1 1/2 hours.
Once the casserole is done, remove from the oven and serve hot as a side dish to any meal.