Preheat your oven to 350°F (175°C). Grease a 9x13-inch baking dish with cooking spray or butter.
In a large mixing bowl, whisk together the eggs, milk, Dijon mustard, garlic powder, salt, black pepper, and nutmeg until well combined.
Layer the torn croissant pieces evenly in the prepared baking dish.
Sprinkle the diced ham, chopped spinach, and shredded cheddar cheese over the croissant pieces.
Pour the egg mixture evenly over the croissant and filling layers, ensuring everything is well soaked.
Cover the dish with plastic wrap and refrigerate for at least 4 hours or overnight for best results.
When ready to bake, remove the casserole from the refrigerator and let it sit at room temperature for about 15 minutes.
Bake in the preheated oven for 35-40 minutes, or until the top is golden brown and the center is set.
Allow to cool for a few minutes before slicing and serving.