Spray a 9x13-inch baking pan with nonstick cooking spray.
Spread the bread cubes across the bottom of the pan. Sprinkle the spinach over the top of the bread, followed by the cheese.
In a mixing bowl, combine the egg, milk, and black pepper. Pour this mixture over the bread mixture. Cover and refrigerate overnight.
In the morning, preheat the oven to 350F. Bake for 35 minutes or until puffy, and a tester inserted comes out clean.
Notes
To freeze: Prior to baking, cover, label, and freeze the casserole. To serve: thaw overnight in the refrigerator before proceeding with the baking instructions.