Spray a 9x9 casserole dish with cooking spray.
If using fresh green beans, clean, cut, and blanch them in boiling water for 2 minutes. If using canned green beans, drain in a colander.
Mix the condensed Cream of Mushroom soup and milk together until smooth. Stir in the soy sauce.
Add green beans and 2/3 cup of the crispy French fried onions (about half). Season with pepper and mix well.
Spread the green bean mixture evenly in the prepared casserole dish.
For Air Fryer: Bake at 350°F for 10 minutes. Stir, then sprinkle remaining onions on top and cook for 2 more minutes until browned.
For Oven: Bake at 350°F for 25 minutes. Stir, then sprinkle remaining onions on top and bake for 5 more minutes until golden brown.