The key to making a great green bean casserole ahead of time is to do some prep work. You can chop the onions, slice the green beans, and even mix the cream of mushroom soup with the milk, all ahead of schedule. Just store them in separate containers in the fridge, and you'll be good to go!
Now, when it comes to assembling the casserole, you can do that up to a few hours in advance. Just mix all the ingredients together, pour them into a baking dish, and refrigerate until you're ready to bake. It's like having a secret ingredient that makes your life easier, don't you think?