If using frozen green beans, thaw them completely and drain any excess liquid; if using fresh beans, trim and cut into 2-inch pieces, then blanch briefly in salted boiling water before draining; canned beans should simply be drained well. Preheat your oven to 350°F (or prepare your air fryer for the same temperature). Measure out your fried onions and divide them into two portions: about 2/3 cup for mixing into the casserole and the remaining 1/3 cup reserved for the crispy topping—I always keep the reserved onions in a small bowl nearby so they stay fresh and don’t absorb any moisture from the air.

In a large mixing bowl, whisk together the condensed cream of mushroom soup, milk, and soy sauce until smooth and well combined. The soy sauce adds a subtle umami depth that enhances the mushroom flavor without making the dish taste salty—whisk it in thoroughly so it distributes evenly throughout the cream base.

Add the drained green beans from Step 1, the 2/3 cup portion of fried onions, and freshly ground black pepper to the soup mixture. Gently fold everything together until the green beans are evenly coated with the creamy sauce and the onions are well distributed throughout. Don’t overmix—you want to keep the fried onions intact so they stay as crispy as possible before the final baking.

Pour the green bean mixture into a 9×9-inch baking dish and spread it into an even layer. Place in your preheated 350°F oven and bake for 25 minutes (or 10 minutes if using an air fryer), until the mixture is bubbling around the edges and heated through.

Remove the casserole from the heat and give it a gentle stir to redistribute the sauce and ensure even cooking. Top with the reserved 1/3 cup of fried onions, spreading them evenly across the surface. Return to the oven and bake for an additional 5 minutes (or 2 minutes in an air fryer) until the onions are golden and the casserole is bubbling gently—this final bake gives you that perfect contrast between the creamy filling and the crispy, warm topping that everyone loves.