Yes! You'll simply want to prepare the casserole and stop before adding the fried onions. Let the green bean mixture cool in the baking dish for 15 minutes, then cover and place in the fridge. It will keep for up to 3 days. When you are ready to serve, uncover the dish and let it sit at room temperature for 30 minutes before adding the onions and baking at 350F (175C) for 30 minutes. You can also bake it directly from the fridge (without thawing first) but it will take longer to bake.