Active time: 30 Minutes, Start to finish: 10 hours (includes chilling)
Squeeze handfuls of spinach to remove liquid. Sauté onion and butter in a large heavy skillet over medium heat, stirring until soft, 4 to 5 minutes. Add ½ teaspoon of salt, ¼ teaspoon of pepper, nutmeg and cook while stirring for 1 minute. Stir in spinach, then remove from heat.Spread one third of bread cubes in a buttered 3-quart baking dish and top evenly with one third of the spinach mixture. Sprinkle with one third of the cheese. Repeat layering twice (ending with cheese).Whisk together milk, eggs, mustard and remaining salt and pepper in a large bowl and pour evenly over strata. Make sure the egg mixture thoroughly soaks the bread otherwise it will turn to toast on top. Cover with plastic wrap and chill at least 8 hours.Let strata stand at room temperature for 30 minutes. Preheat oven to 350°F. Bake strata uncovered in middle rack of oven until puffed and golden brown. Cook through for 45 to 55 minutes. Let stand 5 minutes before serving. Makes 8 servings.