Blanch the green beans in boiling water for 5 minutes. Then, drain the beans and immediately cool them with ice-cold water to stop them from cooking. Transfer the beans to a large mixing bowl.
Pour the green bean casserole filling into a medium-sized casserole dish and spread it out in an even layer. Bake the casserole for 30 minutes, until it's hot and bubbly.
Remove the casserole from the oven, sprinkle the crispy fried onions on top, and return the casserole back into the oven to continue cooking for another 5-10 minutes.