Prepare the Casserole

1. Preheat Oven: Preheat your oven to 350°F (175°C).
2. Cook the Green Beans: In a large pot of boiling salted water, blanch the green beans for about 5 minutes until just tender. Drain and set aside.
3. Mix the Casserole: In a large mixing bowl, combine the cream of mushroom soup, milk, and season with salt and pepper. Stir in the blanched green beans and half of the French fried onions if using.
4. Transfer to Baking Dish: Pour the mixture into a greased 2-quart baking dish, spreading it evenly.

Add the Topping

1. Top with Fried Onions: Scatter the remaining French fried onions evenly over the top of the green bean mixture.

Bake the Casserole

1. Bake: Place the casserole in the preheated oven and bake for 20-25 minutes, or until the casserole is bubbly and the onions are golden brown.