How to Make It
Heat the oven to 350°. Butter an 8-by-12-inch baking dish or a 1 1/2-quart gratin dish. In a large bowl, combine the spinach, artichoke hearts, scallions, bread cubes, and half the mozzarella. Spread this mixture in the bottom of the prepared baking dish. Top with the remaining mozzarella.
In a medium bowl, whisk together the eggs, milk, Parmesan, salt, and pepper. Pour over the vegetables and bread and press the bread into the liquid, making sure that it's well moistened.
Bake the bread pudding for 20 minutes. Raise the oven temperature to 400° and bake until the pudding is puffed and browned, 15 to 20 minutes longer.
Defrost frozen vegetables quickly by removing them from the package and putting them into a bowl of hot water. You may need to squeeze or blot the vegetables dry on paper towels before using them, but the defrosting will take only minutes.