Make Ahead and Reheating Instructions: Bring blanched green beans and mushroom cream sauce onto a casserole dish ahead of time, but only top with fried onions when ready to bake. If green bean casserole was stored in the refrigerator ahead of time, you'll want to bake fresh green bean casserole at 375℉ covered for 20 minutes and uncovered for another 15 minutes to make sure it is heated through.