Cut the ends off the green beans, then cut each bean in half. Boil green beans until somewhat soft, at least 5 minutes at a full boil. Beans should still have a bit of crunch to them, since cooking in the oven will continue to soften them.
Drain beans from hot water and immerse in cold water to stop the cooking process. Drain beans again and set aside.
Saute mushrooms in butter just long enough to soften slightly, then remove from heat.
Season beans with salt and pepper. Layer beans, undiluted mushroom soup, and mushrooms in a casserole pan. Make sure there's a layer of mushroom soup over the top of the beans.
Bake at 350°F for 30 minutes or until hot.
Top with fried onions and cook for another 5 minutes or until golden brown.