I think it would be a gloppy mess. Every few years, I'm the designated GBC maker and I bake it the day before in a microwaveable casserole without the onion ring topping. As dinertime approaches, I heat it in the microwave, then add the onion stuff on top and run it under the broiler after the turkey and other items vacate the oven.

Unfortunately, my extended family also loves GB casserole. I always throw it all in the crockpot. I do mix in the fried onions instead of on top. It is not a gloppy mess.
It is usually in the crockpot for 3-4 hours.

Linda

Linda, do you make any adjustments to the recipe?

I just mix up the green beans with the mushroom soup. I do not add milk to mine. I also crush the fried onions and mix them into the casserole so that they absorb more of the liquid.

Linda

I know everyone has different tolerance for "glop"...But the mere thoughts of putting canned greenbeans canned cream of mushroom soup into a crock pot and cooking it for 3 or 4 hours sounds for sure like it would exceed my tolerance for glop.
Why not bake it ahead and reheat with the onion rings on top? At least the onion rings would be crisp.