To prevent a soggy casserole, try these steps. 1) Use stale bread. 2) Use a fully preheated oven. 3) It is better to use a "too large" baking dish than a "too small" one. The thicker casserole will tend to be more soggy. 4) Do not undercook—better over than under. Once it has been baked but is soggy, you can spread the soggy casserole on a rimmed sheet pan and bake a bit longer.