Preheat the oven to 350 degrees Fahrenheit.
In a microwave-safe bowl, combine the green beans and mushroom soup. Microwave on high power for 3 to 5 minutes, or until warm. Stir in ½ cup of the cheese and microwave for another 2 to 3 minutes.
Transfer the mixture into a casserole and cover with French fried onions and remaining ½ cup of cheese.
Bake the casserole for 5 to 10 minutes, or until the cheese is bubbly and the onions start to brown. Serve and enjoy!
