Grease the bottom and sides of a 9 × 13-inch pan.
In a microwave- safe bowl, cook the spinach on high 2 to 3 minutes, until fully thawed. Let cool for a minute, then squeeze the spinach to drain as much water from it as possible.
Arrange half of the bread in the prepared pan. Scatter half of the spinach and cheeses on top. Repeat with the remaining bread, spinach, and cheeses.
In a large bowl, combine the eggs, milk, salt, and nutmeg and season with pepper. Beat with a whisk or fork until well mixed. Pour over the spinach- cheese mixture. Use a rubber spatula to press the bread down, helping it to soak up the milk mixture. Cover tightly with foil and refrigerate overnight.
The next morning, preheat the oven to 350ºF. Bake the strata, covered, for 30 minutes. Remove the foil and bake for 30 minutes more, until golden and pu¾y (it will deflate a few minutes after removing from the oven). Serve hot.
YOU CAN’T GO WRONG
The strata is a very flexible dish, so feel free to experiment, adding sliced mushrooms sliced cooked sausage, leftover roasted vegetables, or different types of cheese. Go crazy.