01 - Preheat oven to 350°F. Grease a 9x13-inch baking dish with butter.
02 - Arrange torn croissant pieces evenly in the greased baking dish.
03 - In a mixing bowl, combine ricotta, chopped spinach, mozzarella, Parmesan, nutmeg, salt, and pepper. Stir thoroughly until evenly blended.
04 - Spoon the spinach and ricotta mixture evenly over the croissants, gently nestling it between the pieces.
05 - In a separate bowl, whisk eggs with milk, cream, Dijon mustard, and garlic powder until fully combined and smooth.
06 - Pour the egg custard evenly over the croissant and spinach mixture. Press down gently to ensure croissant pieces absorb the liquid.
07 - Allow casserole to rest for 10 minutes at room temperature so croissants soak up custard, or refrigerate covered overnight for enhanced flavor.
08 - Bake uncovered for 35–40 minutes, until the custard is set and the top is golden brown.
09 - Let the bake cool for 10 minutes before slicing and serving.