Prep the veggies: Wash and trim the green beans. Chop the onion and slice the mushrooms.
Sauté: Heat olive oil in a large skillet over medium heat. Add the onion and mushrooms and cook until softened, about 5 minutes. Add the green beans and cook until tender-crisp, about 5-7 minutes more.
Make the sauce: In a large bowl, combine the cream of mushroom soup, milk, and black pepper. Stir in the sautéed vegetables.
Assemble the casserole: Pour the mixture into a greased 9x13 inch baking dish.
Bake: Bake in a preheated oven at 350 degrees F (175 degrees C) for 20 minutes.
Top and bake again: Sprinkle the French fried onions over the top and bake for an additional 5-10 minutes, or until the onions are golden brown and the casserole is heated through.
Watery Casserole Rescue Plan:
Cornstarch Slurry: Whisk together 1 tablespoon cornstarch with 2 tablespoons cold water or broth. Gradually whisk into the casserole while it's simmering on the stovetop. Cook for a few minutes until thickened.
Roux: Melt 1 tablespoon butter in a pan, whisk in 1 tablespoon flour, cook for 1-2 minutes. Slowly whisk the roux into the casserole.
More Soup: Stir in an extra can of cream of mushroom soup.