Lightly grease a microwave-safe casserole dish for easy cleanup.
Layer half of the drained green beans in the bottom of the casserole dish.
Sprinkle half of the French fried onion rings over the green beans.
Repeat with remaining green beans and onion rings to create alternating layers.
Empty the entire can of cream of mushroom soup on top of the casserole.
Spread the soup evenly across the surface using a spoon.
Cover tightly with microwave-safe plastic wrap, turning back one corner slightly to prevent steam buildup.
Microwave on reheat power level for 15 minutes.
Carefully remove plastic wrap, watching for hot steam, and serve immediately.