Heat the oven to 350°F. If preparing the casserole in an air fryer, preheat the appliance to the same temperature.

Prepare 4 cups of green beans according to the type being used. Fully thaw frozen green beans and drain away excess moisture. For fresh beans, trim the ends, cut the beans into approximately 2-inch lengths, briefly blanch them in salted boiling water, and drain thoroughly. Canned green beans only need to be drained well.

Measure the crisp fried onions and separate them into portions. Set aside approximately 2/3 cup for incorporating into the casserole and reserve 1/3 cup for the final topping. Keep the topping portion separate and dry until needed.

Place the condensed cream of mushroom soup, 1/2 cup milk, and 1 teaspoon soy sauce in a large mixing bowl.

Whisk until the ingredients form a smooth, evenly blended sauce. Thorough mixing distributes the soy sauce throughout the mushroom base, adding subtle savory flavor without overwhelming the casserole.

Add the prepared green beans, 2/3 cup fried onions, and 1/4 teaspoon ground black pepper to the creamy mushroom mixture.

Fold the ingredients together gently until the beans are completely coated and the fried onions are distributed throughout the casserole. Avoid excessive stirring so the onions retain as much of their texture as possible before baking.

Transfer the green bean mixture to a 9 × 9-inch baking dish, spreading it into a uniform layer.

Bake in the preheated 350°F oven for 25 minutes, or cook for 10 minutes in the air fryer, until the casserole is thoroughly heated and bubbling around the edges.

Remove the casserole from the heat and stir it gently to redistribute the creamy sauce.

Scatter the reserved 1/3 cup fried onions evenly over the surface.

Return the casserole to the oven for an additional 5 minutes, or to the air fryer for 2 minutes, until the topping is golden and crisp and the filling is gently bubbling.