Make-Ahead Spinach and Mozzarella Breakfast Strata

Melt butter in a medium skillet over medium heat. Add onion and saute until soft, about 5 minutes. Add herbes de Provence, salt, and pepper; continue to cook for 1 minute more. Stir in spinach, remove from heat, and set aside.

Spray the inside of a 3-quart (9x13-inch) baking dish with cooking spray. Layer the bottom of the dish with 1/3 of the bread cubes. Top with 1/3 of the spinach mixture and 1/3 mozzarella cheese. Repeat these layers twice more with remaining bread, spinach mixture, and cheese.

Combine eggs, milk, salt, and pepper in a medium bowl. Whisk together until blended. Pour evenly over the bread and spinach layered in the baking dish. Cover with plastic wrap and chill for at least 8 hours, up to 1 day.

Remove from the refrigerator 30 minutes before baking to allow pan to get to room temperature.

Meanwhile, preheat the oven to 350 degrees F (175 degrees C).

Bake, uncovered, in the preheated oven until puffed, golden brown, and cooked through, 45 to 55 minutes. Let stand at least 5 minutes before serving.