In a medium bowl, mix together green beans, soup, and French fried onions.
Spray down a medium-sized baking sheet with nonstick cooking spray and arrange onion rings so that they are all 1.5-2 inches (4-5 cm) apart.
Using a spoon or spatula, scoop soup/green bean mixture into the middle of each onion ring, making sure to fill each ring evenly before placing another onion ring on top and repeating the process. (NOTE: Make sure to use the largest onion rings first as the base to your stacks, using the smaller rings later towards the top.)
Bake stacks at 350˚F (180˚C) for 18-22 minutes or until onion rings are crisp and golden and green bean mixture is bubbling on top.
Let cool for at least 5 minute before transferring to a plate to serve.