Vegetarian Breakfast Casserole

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.