Preheat oven to 250 degrees.
Cube ciabatta and spread in one layer on a baking sheet. Bake for 20 minutes until lightly toasted and dried out. Set out until cooled to room temperature.
Thaw spinach in microwave and remove all excess liquid. (My handy method is to wrap it in one million paper towels and wring it out.)
Melt 1 tbsp butter in a large skillet over medium heat. Add onion and sauté until soft, about 5 minutes. Add 1/2 teaspoon salt, 1/4 teaspoon pepper and continue cooking for one minute. Stir in spinach, remove from heat and set aside.
Grease baking dish with remaining tablespoon of butter. Spread half of the bread cubes in the dish and top with half of spinach mixture and half of cheese. Repeat layering with remaining bread, spinach and cheese. Whisk eggs, milk, nutmeg, and remaining 1/2 teaspoon salt and 1/4 teaspoon pepper together in a large bowl and pour evenly over the strata.
Cover strata with plastic wrap and refrigerate overnight.
The next day, let it stand at room temperature for 30 minutes while preheating the oven to 350°F. Bake uncovered in the middle of the oven until golden brown and cooked through, 50 minutes. To get it extra crispy on top, broil for 1-2 minutes. Let stand 5 minutes before serving.
Eat, sip your mimosa and help yourself to seconds.