Broccoli is a very forgiving vegetable, which makes freezing it a cinch. A frozen broccoli casserole isn't likely to become waterlogged or mushy when it's thawed, as long as it is handled properly. The key to putting broccoli casserole in the freezer is to make sure the dish is in a freezer-proof container and that it's wrapped very well. This will keep the moisture from building up on the casserole and rendering it soggy and inedible.
When cooking the casserole, go easy on the spices. Broccoli has a strong flavor, especially when reheated. Cut back on the onions, peppers and seasonings in the casserole by about one-fourth to one-third to compensate for the flavor in the thawed broccoli casserole. You can add more salt and pepper if needed after it's reheated.
Prepare the casserole in a dish that is both freezer-proof and oven-proof. Because broccoli casseroles often have cream soups and cheese in them, they will not transfer well from one container to another. This will keep the casserole looking presentable when it's reheated.
Cook the casserole for the full amount of time and allow it to cool completely. It may need to sit out for a few hours, but freezing warm casserole can mean mushy or soggy broccoli when it's thawed.
Cover the casserole dish with a layer of plastic wrap, then a layer of foil. If desired, slide it into a freezer bag for extra protection. Select a level shelf in the freezer; storing the casserole on a level surface will keep it from bubbling or cracking.
Thaw the casserole in the refrigerator overnight before serving, if possible. This will allow the dish to cook more quickly and evenly than baking it frozen, which is particularly important because broccoli burns easily.
Remove the foil and plastic wrap before popping the casserole in the oven. Bake it at the same temperature the recipe originally called for, checking it every 15 or 20 minutes until it's hot in the center. Cover the broccoli casserole with a layer of foil if it appears to be browning too quickly on top.