Preheat oven to 350 degrees F. Spray a 1 ½ -quart baking dish with cooking spray and set aside.
Place frozen green beans in a large microwave-safe bowl. Add a splash of water, cover, and microwave just until crisp-tender (about 5 minutes). Drain and return green beans to the bowl.
Add condensed soup, milk, soy sauce, pepper, and ⅔ cup of the fried onions to the bowl with the green beans; stir to combine.
Transfer the green bean mixture to the prepared baking dish.
Bake, uncovered, for 25 minutes.
Stir, sprinkle remaining ⅔ cup of fried onions on top, and bake for about 5 more minutes.
Swap out the frozen French-cut green beans and use 4 cups of frozen cut green beans, 4 cups of cut fresh green beans, or 2 (14.5 ounce) cans canned green beans (drained). If using frozen or fresh green beans, cook in the microwave until crisp-tender (about 5 minutes for frozen or 8 minutes for fresh).
Instead of the cream of mushroom soup, use a different flavor of condensed soup. Cream of chicken or cream of celery would both be good in this recipe.
Add cooked, chopped bacon to the green bean mixture, or sprinkle cooked bacon on top of the dish during the final few minutes to get nice and crispy.
Sauté sliced fresh mushrooms and stir them into the green bean casserole before baking.
Add 1 cup of grated cheese to the casserole before baking. Sharp cheddar cheese would be perfect!
To serve a larger crowd, double all of the ingredients and bake the casserole in a 3-quart dish. This larger casserole will serve about 12 people. If you need to serve 20 people, I recommend preparing two large casseroles (3 quarts each), which will require 4 times as many ingredients.