If using fresh beans, wash, trim, and cut them into bite-sized pieces. Blanch in boiling water for 2 minutes, then drain well. For canned beans, drain and rinse. For frozen beans, thaw and drain thoroughly.
In a large mixing bowl, whisk together the cream of mushroom soup, milk, and soy sauce until smooth.
Add the green beans and ⅔ cup of crispy fried onions to the sauce mixture. Season with black pepper and stir until evenly coated.
Spray a 9x9-inch baking dish with nonstick spray and spread the mixture evenly inside.
Oven: Preheat to 350°F (175°C) and bake for 25 minutes.
Air Fryer: Cook at 350°F for 10 minutes, then stir.
Sprinkle remaining onions evenly over the top.
Oven: Return for 5 more minutes until golden.
Air Fryer: Cook 2 more minutes until crisp.
Let the casserole cool slightly before serving. Add extra onions on top if you love more crunch.