Preheat oven to 350°F. Spray an 8x8 inch oven-safe dish with cooking spray.
Drain the green beans and place in a large bowl. Add ¾ cup of french fried onions, and toss gently to combine.
In a small saucepan, heat the mushroom soup and milk over medium heat for 2 minutes. Whisk to combine. Pour the mixture over the green bean mixture and toss gently to combine.
Place the mixture into the prepared dish.
Bake for 20 minutes.
Sprinkle the remaining fried onions over the top (as many as you’d like—you may not need them all). Bake for 5 additional minutes. Serve while warm.
You can use cream of celery soup in place of the cream of mushroom soup.You can substitute 18 ounces of frozen green beans (thawed) in place of the canned green beans. I used a 10.5x7 inch dish for this recipe, which is approximately the same size as an 8x8 inch dish. Either works well. Make it ahead: follow instructions through step #4. Cover the dish tightly with plastic wrap and refrigerate for up to 24 hours. When you're ready to bake it, follow the instructions as written in the recipe.Reheating instructions: place the casserole dish with any leftovers in a preheated 350°F oven. Bake for 10–15 minutes or until warm. You may need to cover the top with foil after 5–10 minutes to prevent the fried onions from burning. I do not recommend freezing this dish; it will compromise the texture of the fried onions.