Preheat the oven to 350 degrees F (175 degrees C).

Heat a large saucepan of salted water to a boil; add green beans and cook just until slightly tender, about 5 minutes. Drain.

Mix green beans and cream of mushroom soup together in a bowl; spread into a 9x13-inch baking dish. Sprinkle Cheddar cheese over the green bean mixture and top with French-fried onions.

Bake in the preheated oven until cheese is melted and browned, about 30 minutes.