1. Prepare the recipe the night before serving, and let sit overnight in the refrigerator.
2. If using sausage, cook as directed on box and crumble.
3. Place crumbled sausage (if using), eggs, milk, and salt in a 9 x 13 pan, and mix together with a fork.
4. Add zucchini, onion, mushrooms, spinach, and cheese, and gently mix into egg mixture.
5. Add cubed bread, and gently fold all ingredients together until uniformly mixed.
6. Cover and let sit in refrigerator overnight.
7. The day that you want to eat the casserole, pull the casserole out of the refrigerator and preheat the oven to 350 degrees. Once preheated, bake the casserole for 1 hour to 1 hour and 15 minutes (or until knife comes out clean). Serve hot and enjoy.