Tip 1. Remove Extra Moisture from Wet Vegetables Before Cooking.
If you don’t want to end up with a swimming pool at the bottom of your casserole, you may have to give certain vegetables some TLC before they hit the baking dish. Frozen vegetables can be particularly problematic. Thaw them in a strainer so they lose their extra liquid. You can squeeze moisture out of frozen greens like spinach to dry them out. Also be wary of mushrooms: they’re mostly water and need to be precooked.