There are a few reasons why your casserole may be dry. The most common mistake is not adding enough liquid. Do not skip stirring in milk to the cream of mushroom as the egg noodles soak up the liquid, keeping the casserole moist. Also, I recommend setting a timer for your oven, as overbaking will dry the casserole out. Finally, make sure you’re using the correct sized baking dish for your casserole. If it’s too large, the casserole will bake through faster, leading to a dry dish.