01 - Preheat oven to 350°F. Grease a 9x13-inch baking dish with unsalted butter.
02 - Layer torn croissant pieces evenly in the buttered baking dish.
03 - In a mixing bowl, blend ricotta, spinach, mozzarella, Parmesan, nutmeg, salt, and black pepper until combined.
04 - Distribute the spinach and ricotta mixture evenly over croissants, gently nestling between the pieces.
05 - In a separate bowl, whisk eggs, whole milk, heavy cream, Dijon mustard, and garlic powder until smooth.
06 - Pour egg custard evenly over the croissant and spinach layers, then press down gently to ensure croissants absorb the liquid.
07 - Allow the assembled bake to rest for 10 minutes for absorption, or refrigerate covered overnight to intensify flavor.
08 - Bake uncovered for 35 to 40 minutes, until the custard is set and the top is golden brown.
09 - Cool for 10 minutes before portioning and serving.