"This is a delicious breakfast casserole that has lots of vegetables and is meat-free. I make this for guests, potluck brunches, and Christmas morning. This recipe is very flexible. Feel free to use any type of bread or cheese you have on hand, and add or remove vegetables to suit your preferences. I recommend serving with fruit for a nice balanced breakfast! Don't worry if the recipe makes too much, just reheat in the microwave; it's still good for breakfast or lunch the next day."
Heat a skillet over medium heat and spray with cooking spray. Place frozen sausage patties in the skillet. Cook for 5 minutes; turn over and cook until heated through, about 5 minutes more. Transfer patties to a 9x13-inch baking pan and break them up with a fork.
Mix eggs, milk, and salt into the baking pan with the sausage. Add spinach, Cheddar cheese, mushrooms, zucchini, and onion. Add bread and fold all ingredients together until uniformly mixed. Cover casserole with plastic wrap and refrigerate, 8 hours to overnight.
Preheat oven to 350 degrees F (175 degrees C).
Bake the casserole in the preheated oven until eggs are set and a knife inserted in the center comes out clean, 1 hour to 1 hour and 15 minutes.