Preheat the oven to 360 F. Fry the mushrooms in butter for 5-7 minutes on the stove to soften a bit. Add the frozen green beans to a bowl, as well as the sautéed mushrooms and 1/4 cup of crispy onions.
Add 2/3 cup cheese and mushroom soup. Mix thoroughly and transfer the mixture to a sprayed square baking dish. Bake for 20 minutes. Sprinkle on top the remaining cheese and remaining crispy onions on top and bake for another 10 minutes.